View source for Biology:2009 swine flu pandemic

{{short description|2009–2010 pandemic of swine influenza caused by H1N1 influenza virus}}
{{Infobox pandemic
| name            = 2009 swine flu pandemic
| width           = 
| map1            = H1N1 map by confirmed cases.svg{{!}}upright=1.25
| legend1         = {{legend|#2b0000|50,000+ confirmed cases}}{{legend|#550000|5,000–49,999 confirmed cases}}{{legend|#8c0000|500–4,999 confirmed cases}}{{legend|#d40000|50–499 confirmed cases}}{{legend|#ff6666|5–49 confirmed cases}}{{legend|#ff9999|1–4 confirmed cases}}{{legend|#b9b9b9|No confirmed cases}}
| map2            = {{multiple image|border=infobox|perrow=1/2/2/2|total_width=305
| image1 = Mexican Catholics at the Metropolitan Cathedral Swine Flu.jpg
| image2 = St Gabriel flu outbreak.png
| image3 = Apropiación Appropriation (3492602261).jpg
| image4 = Possible Swine Flu Victim Mexico City.jpg
| image5 = Swine flu in Kazakhstan (2009).jpg
| image6 = Pandemrix3.jpg
}}
| disease         = [[Medicine:Influenza|Influenza]]
| virus_strain    = Pandemic H1N1/09 virus
| location        = Worldwide
| arrival_date    = September 2008<ref name="Nature" /><ref name="circles globe" />
| date            = January 2009 – 10 August 2010<ref name="Reuters6-11" /><ref name="WHO declares H1N1 Pandemic over" />
| source          = Pigs, likely indirectly
| first_case      = Veracruz, Mexico<ref name= "CDCmmw4-09">{{Cite web|url = https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5817a5.htm |title=Outbreak of Swine-Origin Influenza A (H1N1) Virus Infection: Mexico, March–April 2009 | date=30 April 2009 |website=The Centers for Disease Control and Prevention (CDC)|url-status=live|archive-url= https://web.archive.org/web/20200320042228/https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5817a5.htm/|archive-date=20 March 2020|access-date=20 March 2020}}</ref><ref>{{cite journal | vauthors = Perez-Padilla R, de la Rosa-Zamboni D, Ponce de Leon S, Hernandez M, Quiñones-Falconi F, Bautista E, Ramirez-Venegas A, Rojas-Serrano J, Ormsby CE, Corrales A, Higuera A, Mondragon E, Cordova-Villalobos JA | display-authors = 6 | title = Pneumonia and respiratory failure from swine-origin influenza A (H1N1) in Mexico | journal = The New England Journal of Medicine | volume = 361 | issue = 7 | pages = 680–89 | date = August 2009 | pmid = 19564631 | doi = 10.1056/NEJMoa0904252 | author14 = INER Working Group on Influenza | doi-access = free }}</ref>
| origin          = [[Earth:North America|North America]]<ref name="CDCmmw4-09" /><ref>{{Cite web|url=https://www.cdc.gov/h1n1flu/information_h1n1_virus_qa.htm|title=Origin of 2009 H1N1 Flu (Swine Flu): Questions and Answers | date=25 November 2009|website=Centers for Disease Control and Prevention (CDC)|url-status=live|archive-url=https://web.archive.org/web/20200318172324/https://www.cdc.gov/h1n1flu/information_h1n1_virus_qa.htm|archive-date=18 March 2020|access-date=20 March 2020}}</ref>
| confirmed_cases = 491,382 (lab-confirmed)<ref name="WHO99">{{Cite web|url= https://www.who.int/csr/disease/swineflu/laboratory06_08_2010/en/|archive-url= https://web.archive.org/web/20150807183818/http://www.who.int/csr/disease/swineflu/laboratory06_08_2010/en/|archive-date= 7 August 2015|title=Weekly Virological Update on 05 August 2010 | date=5 August 2010|website=World Health Organization (WHO) |access-date=8 April 2020}}</ref>
| suspected_cases = 700 million to 1.4 billion (estimate)<ref name="cases" />
| recovery_cases  = 
| deaths          = Lab confirmed deaths: 18,449 (reported to the [[Organization:World Health Organization|WHO]])<ref name="WHO100">{{Cite web|url= https://www.who.int/csr/don/2010_08_06/en/|archive-url= https://web.archive.org/web/20110327194118/http://www.who.int/csr/don/2010_08_06/en/|archive-date= 27 March 2011|title=Pandemic (H1N1) 2009 | date=6 August 2010|website=World Health Organization (WHO) |access-date=8 April 2020}}</ref><br />Estimated excess death: 284,000<ref>{{cite web |title=CDC H1N1 Flu {{!}} Influenza Diagnostic Testing During the 2009–2010 Flu Season |url=https://www.cdc.gov/h1n1flu/diagnostic_testing_public_qa.htm |website=www.cdc.gov |access-date=4 April 2020 |archive-date=26 November 2009 |archive-url=https://web.archive.org/web/20091126004503/http://www.cdc.gov/h1n1flu/diagnostic_testing_public_qa.htm |url-status=live }}</ref>
| territories     = 
| total_ili       = 
| website         = 
}}
{{flu}}

The '''2009 swine flu pandemic''', caused by the [[Biology:Influenza A virus subtype H1N1|H1N1/swine flu/influenza virus]] and declared by the [[Organization:World Health Organization|World Health Organization]] (WHO) from June 2009 to August 2010, was the third recent flu pandemic involving the H1N1 virus (the first being the 1918–1920 Spanish flu pandemic and the second being the 1977 Russian flu).<ref name=":02">{{Cite web|date=April 1999|title=Influenza Pandemic Plan. The Role of WHO and Guidelines for National and Regional Planning|url=https://www.who.int/csr/resources/publications/influenza/whocdscsredc991.pdf|url-status=live|archive-url=https://web.archive.org/web/20201203084733/https://www.who.int/csr/resources/publications/influenza/whocdscsredc991.pdf|archive-date=3 December 2020|website=[[Organization:World Health Organization|World Health Organization]]|pages=38, 41}}</ref><ref name=":6">{{cite journal|vauthors=Michaelis M, Doerr HW, Cinatl J|date=August 2009|title=Novel swine-origin influenza A virus in humans: another pandemic knocking at the door|journal=Medical Microbiology and Immunology|volume=198|issue=3|pages=175–83|doi=10.1007/s00430-009-0118-5|pmid=19543913|s2cid=20496301|doi-access=free}}</ref> The first known human case was in La Gloria, Mexico, a rural town in Veracruz.<ref name=":0" /><ref name="NYT6-23" /> The virus appeared to be a new strain of H1N1 that resulted from a previous triple [[Biology:Reassortment|reassortment]] of bird, swine, and human flu viruses which further combined with a Eurasian pig flu virus,<ref>{{cite journal | vauthors = Trifonov V, Khiabanian H, Rabadan R | s2cid = 205105276 | title = Geographic dependence, surveillance, and origins of the 2009 influenza A (H1N1) virus | journal = The New England Journal of Medicine | volume = 361 | issue = 2 | pages = 115–19 | date = July 2009 | pmid = 19474418 | doi = 10.1056/NEJMp0904572 | df = dmy-all | doi-access = free }}</ref> leading to the term "swine flu".<ref>{{Cite news | vauthors = Hellerman C |url= http://www.cnn.com/2009/HEALTH/06/11/swine.flu.who/ |title= Swine flu 'not stoppable,' World Health Organization says |publisher= CNN |date=11 June 2009 |access-date= 3 April 2010 |url-status= live |archive-url= https://web.archive.org/web/20100307163916/http://www.cnn.com/2009/HEALTH/06/11/swine.flu.who/ |archive-date=7 March 2010 |df= dmy-all}}</ref>

Some studies estimated that the real number of cases including asymptomatic and mild cases could be 700 million to 1.4 billion people—or 11 to 21 percent of the global population of 6.8 billion at the time.<ref name="cases">{{cite web|url=http://www.cidrap.umn.edu/news-perspective/2011/08/study-puts-global-2009-h1n1-infection-rate-11-21|title=Study puts global 2009 H1N1 infection rate at 11% to 21%|vauthors=Roos R|date=8 August 2011|publisher=Center for Infectious Disease Research and Policy|access-date=25 January 2020|archive-date=25 November 2020|archive-url=https://web.archive.org/web/20201125151837/https://www.cidrap.umn.edu/news-perspective/2011/08/study-puts-global-2009-h1n1-infection-rate-11-21|url-status=live}}</ref> The lower value of 700 million is more than the 500 million people estimated to have been infected by the Spanish flu pandemic.<ref name="age-specific cumulative incidence">{{cite journal | vauthors = Kelly H, Peck HA, Laurie KL, Wu P, Nishiura H, Cowling BJ | title = The age-specific cumulative incidence of infection with pandemic influenza H1N1 2009 was similar in various countries prior to vaccination | journal = PLOS ONE | volume = 6 | issue = 8 | pages = e21828 | date = 5 August 2011 | pmid = 21850217 | pmc = 3151238 | doi = 10.1371/journal.pone.0021828 | bibcode = 2011PLoSO...621828K | doi-access = free }}</ref> However, the Spanish flu infected approximately a third of the world population at the time, a much higher proportion.<ref name="brutal">{{cite web |vauthors=Hagemann H |title=The 1918 Flu Pandemic Was Brutal, Killing More Than 50 Million People Worldwide |url=https://www.npr.org/2020/04/02/826358104/the-1918-flu-pandemic-was-brutal-killing-as-many-as-100-million-people-worldwide |website=NPR |access-date=24 July 2020 |date=2 April 2020 |archive-date=19 January 2021 |archive-url=https://web.archive.org/web/20210119213925/https://www.npr.org/2020/04/02/826358104/the-1918-flu-pandemic-was-brutal-killing-as-many-as-100-million-people-worldwide |url-status=live }}</ref>

The number of lab-confirmed deaths reported to the WHO is 18,449<ref name="WHO100"/> and is widely considered a gross underestimate.<ref>{{Cite web|last=WHO|date=|title=Summary of WHO Technical Consultation: H1N1pdm Mortality Estimates, 25-26 October 2011|url=https://www.who.int/influenza/publications/surveillance_monitoring/H1N1pdmMortalityEstimates_WHOconsultation.pdf|access-date=1 November 2021|archive-date=2 November 2021|archive-url=https://web.archive.org/web/20211102220723/https://www.who.int/influenza/publications/surveillance_monitoring/H1N1pdmMortalityEstimates_WHOconsultation.pdf|url-status=live}}</ref> The WHO collaborated with the US Centers for Disease Control and Prevention (USCDC) and Netherlands Institute for Health Services Research (NIVEL) to produce two independent estimates of the influenza deaths that occurred during the global pandemic using two distinct methodologies. The 2009 H1N1 flu pandemic is estimated to have actually caused about 284,000 (range from 150,000 to 575,000) excess deaths by the WHO-USCDC study and 148,000–249,000 excess respiratory deaths by the WHO-NIVEL study.<ref name= "CDCDeathtolls">{{cite web |title=First Global Estimates of 2009 H1N1 Pandemic Mortality Released by CDC-Led Collaboration |url=https://www.cdc.gov/flu/spotlights/pandemic-global-estimates.htm |website=cdc.gov |access-date=31 January 2020 |ref=CDCDeathtollsNov2019 |date=25 June 2012 |archive-date=3 March 2020 |archive-url=https://web.archive.org/web/20200303012120/https://www.cdc.gov/flu/spotlights/pandemic-global-estimates.htm |url-status=live }}</ref><ref>{{cite journal | vauthors = Simonsen L, Spreeuwenberg P, Lustig R, Taylor RJ, Fleming DM, Kroneman M, Van Kerkhove MD, Mounts AW, Paget WJ | display-authors = 6 | title = Global mortality estimates for the 2009 Influenza Pandemic from the GLaMOR project: a modeling study | journal = PLOS Medicine | volume = 10 | issue = 11 | pages = e1001558 | date = November 2013 | pmid = 24302890 | pmc = 3841239 | doi = 10.1371/journal.pmed.1001558 | doi-access = free }}</ref> A study done in September 2010 showed that the risk of serious illness resulting from the 2009 H1N1 flu was no higher than that of the yearly [[Biology:Flu season|seasonal flu]].<ref>{{cite web |vauthors=DeNoon DJ |title=H1N1 Swine Flu No Worse Than Seasonal Flu |url=https://www.webmd.com/cold-and-flu/news/20100907/h1n1-swine-flu-no-worse-than-seasonal-flu |website=WebMD |access-date=13 March 2020 |archive-date=17 October 2020 |archive-url=https://web.archive.org/web/20201017085947/https://www.webmd.com/cold-and-flu/news/20100907/h1n1-swine-flu-no-worse-than-seasonal-flu |url-status=live }}</ref> For comparison, the WHO estimates that 250,000 to 500,000 people die of seasonal flu annually.<ref name="death toll estimate">{{cite web |date=27 June 2012 |vauthors=Roos R |title=CDC estimate of global H1N1 pandemic deaths: 284,000 |url=http://www.cidrap.umn.edu/news-perspective/2012/06/cdc-estimate-global-h1n1-pandemic-deaths-284000 |website=CIDRAP |access-date=23 March 2020 |archive-date=19 September 2020 |archive-url=https://web.archive.org/web/20200919203235/https://www.cidrap.umn.edu/news-perspective/2012/06/cdc-estimate-global-h1n1-pandemic-deaths-284000 |url-status=live }}</ref> However, the H1N1 influenza epidemic in 2009 resulted in a large increase in the number of new cases of [[Medicine:Narcolepsy|narcolepsy]].<ref>{{cite web |title=Narcolepsy Fact Sheet |url=https://www.ninds.nih.gov/narcolepsy-fact-sheet |website=Liaison National Institute of Neurological Disorders and Stroke |access-date=23 September 2020 |archive-date=27 July 2016 |archive-url=https://web.archive.org/web/20160727232102/http://www.ninds.nih.gov/disorders/narcolepsy/detail_narcolepsy.htm |url-status=live }}</ref>

Unlike most strains of influenza, the pandemic H1N1/09 virus did not disproportionately infect adults older than 60&nbsp;years; this was an unusual and characteristic feature of the H1N1 [[Finance:Pandemic|pandemic]].<ref>{{cite journal | vauthors = Bautista E, Chotpitayasunondh T, Gao Z, Harper SA, Shaw M, Uyeki TM, Zaki SR, Hayden FG, Hui DS, Kettner JD, Kumar A, Lim M, Shindo N, Penn C, Nicholson KG | display-authors = 6 | title = Clinical aspects of pandemic 2009 influenza A (H1N1) virus infection | journal = The New England Journal of Medicine | volume = 362 | issue = 18 | pages = 1708–19 | date = May 2010 | pmid = 20445182 | doi = 10.1056/NEJMra1000449 | hdl = 2381/15212 | hdl-access = free }}</ref> Even in the case of previously healthy people, a small percentage develop [[Medicine:Pneumonia|pneumonia]] or [[Medicine:Acute respiratory distress syndrome|acute respiratory distress syndrome]] (ARDS). This manifests itself as increased breathing difficulty and typically occurs three to six days after initial onset of flu symptoms.<ref name= "WHObriefingnoteOct16">{{Cite news
|url=https://www.who.int/csr/disease/swineflu/notes/h1n1_clinical_features_20091016/en/ |title= Clinical features of severe cases of pandemic influenza |date= 16 October 2009 |publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date= 25 October 2009 |location= Geneva, {{wipe|Switzerland}} | archive-url= https://web.archive.org/web/20091025105555/http://www.who.int/csr/disease/swineflu/notes/h1n1_clinical_features_20091016/en/ | archive-date= 25 October 2009 }}</ref><ref name= RongGongLinNov21>{{Cite news|url= http://www.latimes.com/features/health/la-me-flu-guidelines21-2009nov21,0,2717012.story |title=When to take a sick child to the ER |newspaper={{wipe|Los Angeles Times}} | vauthors = Lin II RG |date= 21 November 2009 |access-date=4 January 2010 |archive-url= https://web.archive.org/web/20091125135818/http://www.latimes.com/features/health/la-me-flu-guidelines21-2009nov21%2C0%2C2717012.story |archive-date=25 November 2009 |url-status= live}}</ref> The pneumonia caused by flu can be either direct [[Medicine:Viral pneumonia|viral pneumonia]] or a secondary [[Medicine:Bacterial pneumonia|bacterial pneumonia]]. A November 2009 ''New England Journal of Medicine'' article recommended that flu patients whose chest X-ray indicates pneumonia receive both antivirals and [[Chemistry:Antibiotic|antibiotic]]s.<ref>{{cite journal | vauthors = Jain S, Kamimoto L, Bramley AM, Schmitz AM, Benoit SR, Louie J, Sugerman DE, Druckenmiller JK, Ritger KA, Chugh R, Jasuja S, Deutscher M, Chen S, Walker JD, Duchin JS, Lett S, Soliva S, Wells EV, Swerdlow D, Uyeki TM, Fiore AE, Olsen SJ, Fry AM, Bridges CB, Finelli L | display-authors = 6 | title = Hospitalized patients with 2009 H1N1 influenza in the United States, April–June 2009 | journal = The New England Journal of Medicine | volume = 361 | issue = 20 | pages = 1935–44 | date = November 2009 | pmid = 19815859 | doi = 10.1056/NEJMoa0906695 | citeseerx = 10.1.1.183.7888 }} This study involved a total of 272 patients, which represents approximately 25% of US hospitalized patients with lab-confirmed H1N1 whose cases were reported to the US Centers for Disease Control and Prevention (CDC) from 1{{nbsp}}May to 9{{nbsp}}June 2009. The study found that "the only variable that was significantly associated with a positive outcome was the receipt of antiviral drugs within two days after the onset of illness" [Outcomes section, 2nd paragraph] and also that "only 73% of patients with radiographic evidence of pneumonia received antiviral drugs, whereas 97% received antibiotics." [Discussion section, 8th paragraph]. It is recommended that such patients receive both.</ref> In particular, it is a warning sign if a child seems to be getting better and then relapses with high fever, as this relapse may be bacterial pneumonia.<ref name= DeniseGradySept3/>

==Name==


The [[Organization:World Health Organization|World Health Organization]] uses the term "(H1N1) 2009 pandemic" when referring to the event, and officially adopted the name "A(H1N1)pdm09" for the virus in 2010, after the conclusion of the pandemic.<ref>{{Cite news|url=https://www.who.int/influenza/gisrs_laboratory/terminology_ah1n1pdm09/en/|archive-url=https://web.archive.org/web/20120505042135/http://www.who.int/influenza/gisrs_laboratory/terminology_ah1n1pdm09/en/|archive-date=5 May 2012|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO)|access-date=31 March 2020|title=Standardization of terminology of the pandemic A(H1N1)2009 virus}}</ref>

Controversy arose early on regarding the wide assortment of terms used by journalists, academics,
and officials. Labels like "H1N1 flu", "Swine flu", "Mexican flu", and variations thereof were typical. Criticism centered on how these names may confuse or mislead the public. It was argued that the names were [[Social:Jargon|overly technical]] (e.g. "H1N1"), incorrectly implying that the disease is caused by contact with pigs or pig products, or provoking stigmatization against certain communities (e.g. "Mexican"). Some academics of the time asserted there is nothing wrong with such names,<ref>{{cite journal | vauthors = de Lamballerie X, Gould EA | title = With reference to Mexican flu | journal = Influenza and Other Respiratory Viruses | volume = 3 | issue = 5 | pages = 203 | date = September 2009 | pmid = 21462392 | pmc = 4941550 | doi = 10.1111/j.1750-2659.2009.00097.x }}</ref> while research published years later (in 2013) concluded that Mexican Americans and Latino Americans had indeed been stigmatized due to the frequent use of term "Mexican flu" in the news media.<ref>{{cite journal | vauthors = McCauley M, Minsky S, Viswanath K | title = The H1N1 pandemic: media frames, stigmatization and coping | journal = BMC Public Health | volume = 13 | pages = 1116 | date = December 2013 | pmid = 24299568 | pmc = 3907032 | doi = 10.1186/1471-2458-13-1116 | doi-access = free }}</ref>

Official entities adopted terms with varying consistency over the course of the pandemic. The CDC used names like "novel influenza A (H1N1)" or "2009 H1N1 flu".<ref>{{cite web|url=https://www.cdc.gov/h1n1flu/guidance/cruiseships.htm|title=Interim Novel Influenza A (H1N1) Guidance for Cruise Ships |date=5 August 2009|publisher=Centers for Disease Control and Prevention (CDC) |access-date=30 September 2009| archive-url= https://web.archive.org/web/20091001112752/http://www.cdc.gov/h1n1flu/guidance/cruiseships.htm| archive-date= 1 October 2009 | url-status= live}}</ref> The Netherlands National Institute for Public Health and the Environment used the term "Pig Flu" early on. Officials in Taiwan suggested use of the names "H1N1 flu" or "new flu".<ref>{{Cite news |url=http://www.chinapost.com.tw/health/infectious-diseases/2009/04/29/206057/p2/Renamed-swine.htm |title=Renamed swine flu certain to hit Taiwan |date=28 April 2009 |work=The China Post |location=Taiwan (ROC) |access-date=26 September 2009 |url-status=live |archive-url=https://web.archive.org/web/20091007163957/http://www.chinapost.com.tw/health/infectious-diseases/2009/04/29/206057/p2/Renamed-swine.htm |archive-date=7 October 2009 }}</ref> The World Organization for Animal Health, an [[Social:Intergovernmental organization|IGO]] based in Europe, proposed the name "North American influenza".<ref name="NYT 2009-04-28">{{Cite news |url= https://www.nytimes.com/2009/04/29/world/asia/29swine.html |title=The naming of swine flu, a curious matter | vauthors = Bradsher K |date=28 April 2009 |work=The New York Times |access-date=29 April 2009| archive-url= https://web.archive.org/web/20110204092042/http://www.nytimes.com/2009/04/29/world/asia/29swine.html| archive-date=4 February 2011| url-status= live}}</ref> The European Commission adopted the term "novel flu virus". Officials in [[Place:Israel|Israel]] and {{wipe|South Korea}} briefly considered adoption of the name "Mexican virus" due to concern about the use of the word "swine".<ref name="mexflu">{{Cite news|url=http://www.morungexpress.com/analysis/27870.html|title=Swine Flu: A pandemic| vauthors = Kikon CS |work=The Morung Express|access-date=28 September 2009 |archive-url= https://web.archive.org/web/20110714120650/http://www.morungexpress.com/analysis/27870.html |archive-date=14 July 2011}}</ref> In Israel, objections stemmed from sensitivity to religious restrictions on eating pork in the Jewish and [[Religion:Muslim|Muslim]] populations,<ref>{{cite news|title=What's in a name? Governments debate 'swine flu' versus 'Mexican' flu|vauthors=Pilkington E|date=28 April 2009|url=https://www.theguardian.com/world/2009/apr/28/mexican-swine-flu-pork-name|access-date=10 December 2016|archive-date=1 May 2009|archive-url=https://web.archive.org/web/20090501104953/http://www.guardian.co.uk/world/2009/apr/28/mexican-swine-flu-pork-name|url-status=live}}</ref> in {{wipe|South Korea}}, concerns were influenced by the importance of pork and [[Biology:Domestic pig|domestic pig]]s.

As terminology changed to deal with these and other such issues, further criticism was made that the situation was unnecessarily confusing. For example, the news department at the journal ''Science'' produced an article with the humorous title "Swine Flu Names Evolving Faster Than Swine Flu Itself".<ref>{{cite news|title=Swine Flu Names Evolving Faster Than Swine Flu Itself|vauthors=Enserink M|date=8 May 2009|url=https://www.science.org/content/article/swine-flu-names-evolving-faster-swine-flu-itself|publisher=Science|access-date=30 June 2022|archive-date=25 April 2022|archive-url=https://web.archive.org/web/20220425153811/https://www.science.org/content/article/swine-flu-names-evolving-faster-swine-flu-itself|url-status=live}}</ref>

==History==

Analysis of the genetic divergence of the virus in samples from different cases indicated that the virus jumped to humans in 2008, probably after June, and not later than the end of November,<ref name=NsFirstJumped>{{cite journal | vauthors = MacKenzie D | url = https://www.newscientist.com/article/mg20227073.500-swine-flu-can-science-save-us-from-the-second-wave.html | title = Swine flu: Can science save us from the second wave? | journal = New Scientist | date = 9 May 2009 | issue = 2707 | pages = 4–5 | archive-url = https://web.archive.org/web/20090509093515/http://www.newscientist.com/article/mg20227073.500-swine-flu-can-science-save-us-from-the-second-wave.html | archive-date = 9 May 2009 | doi = 10.1016/S0262-4079(09)61215-X }} [http://hxnxflu.blogspot.com/2009/05/can-science-save-us-from-second-wave.html Alt URL] {{Webarchive|url=https://web.archive.org/web/20200425092922/http://hxnxflu.blogspot.com/2009/05/can-science-save-us-from-second-wave.html |date=25 April 2020 }}</ref> likely around September 2008.<ref name="Nature" /><ref name="circles globe" /> The research also indicated the virus had been latent in pigs for several months prior to the outbreak, suggesting a need to increase agricultural surveillance to prevent future outbreaks.<ref name=OxfordU>{{cite journal |vauthors=Smith GJ, Vijaykrishna D, Bahl J, Lycett SJ, Worobey M, Pybus OG, Ma SK, Cheung CL, Raghwani J, Bhatt S, Peiris JS, Guan Y, Rambaut A |display-authors=6 |title=Origins and evolutionary genomics of the 2009 swine-origin H1N1 influenza A epidemic |journal=Nature |volume=459 |issue=7250 |pages=1122–25 |date=June 2009 |pmid=19516283 |doi=10.1038/nature08182 |bibcode=2009Natur.459.1122S |doi-access=free}}
*{{lay source |template=cite web |date=11 June 2009 |title=Swine flu origins revealed |url=http://www.ox.ac.uk/media/news_stories/2009/090611_1.html |archive-url=https://web.archive.org/web/20130425212949/http://www.ox.ac.uk/media/news_stories/2009/090611_1.html |archive-date=25 April 2013 |website=University of Oxford}}</ref> In 2009, U.S. agricultural officials speculated, although emphasizing that there was no way to prove their hypothesis, that "contrary to the popular assumption that the new swine flu pandemic arose on factory farms in Mexico, [the virus] most likely emerged in pigs in Asia, but then traveled to North America in a human."<ref name="NYT6-23">{{Cite news | vauthors = McNeil Jr DG | title = In New Theory, Swine Flu Started in Asia, Not Mexico | work = The New York Times | date = 23 June 2009 | url = https://www.nytimes.com/2009/06/24/health/24flu.html | access-date = 1 September 2009 | url-status = live | archive-url = https://web.archive.org/web/20090807164633/http://www.nytimes.com/2009/06/24/health/24flu.html | archive-date = 7 August 2009 | df = dmy-all }}</ref> However, a subsequent report<ref>{{cite journal | vauthors = Mena I, Nelson MI, Quezada-Monroy F, Dutta J, Cortes-Fernández R, Lara-Puente JH, Castro-Peralta F, Cunha LF, Trovão NS, Lozano-Dubernard B, Rambaut A, van Bakel H, García-Sastre A | display-authors = 6 | title = Origins of the 2009 H1N1 influenza pandemic in swine in Mexico | journal = eLife | volume = 5 | pages = e16777 | date = June 2016 | pmid = 27350259 | pmc = 4957980 | doi = 10.7554/eLife.16777 | doi-access = free }}</ref> by researchers at the [[Organization:Icahn School of Medicine at Mount Sinai|Mount Sinai School of Medicine]] in 2016 found that the 2009 H1N1 virus likely originated from pigs in a very small region of central Mexico.<ref name="swine flu originated in Mexico">{{cite web|url=https://www.sciencedaily.com/releases/2016/06/160627160935.htm|title=2009 swine flu pandemic originated in Mexico, researchers discover|publisher=Science Daily|date=27 June 2016|access-date=18 March 2020|archive-date=14 March 2020|archive-url=https://web.archive.org/web/20200314073206/https://www.sciencedaily.com/releases/2016/06/160627160935.htm|url-status=live}}</ref>

[[File:2009 Mexican military giving out swine flu masks.jpg|thumb|right|Mexican soldiers distributing protective masks to citizens]]
Initially called an "outbreak", widespread H1N1 infection was first recognized in the state of Veracruz, Mexico, with evidence that the virus had been present for months before it was officially called an "epidemic".<ref name="NYT6-23"/> The Mexican government closed most of [[Place:Mexico City|Mexico City]]'s public and private facilities in an attempt to contain the spread of the virus; however, it continued to spread globally, and clinics in some areas were overwhelmed by infected people. The new virus was first isolated in late April by American and Canadian laboratories from samples taken from people with flu in Mexico, Southern California, and Texas. Soon the earliest known human case was traced to a case from 9{{nbsp}}March 2009 in a 5-year-old boy in La Gloria, Mexico, a rural town in Veracruz.<ref name=":0">{{Cite news|vauthors=Lacey M|url=https://www.nytimes.com/2009/04/29/world/americas/29mexico.html|title=From Édgar, 5, Coughs Heard Round the World|date=28 April 2009|work=The New York Times|access-date=21 February 2020|issn=0362-4331|archive-date=2 December 2020|archive-url=https://web.archive.org/web/20201202014813/https://www.nytimes.com/2009/04/29/world/americas/29mexico.html|url-status=live}}</ref><ref name="NYT6-23"/> In late April, the [[Organization:World Health Organization|World Health Organization]] (WHO) declared its first ever "public health emergency of international concern", or PHEIC,<ref>{{cite web|url=https://www.who.int/mediacentre/news/statements/2009/h1n1_20090425/en/|title=WHO – Swine influenza|archive-url=https://web.archive.org/web/20140506113902/http://www.who.int/mediacentre/news/statements/2009/h1n1_20090425/en/|archive-date=6 May 2014}}</ref> and in June, the WHO and the U.S. CDC stopped counting cases and declared the outbreak a [[Finance:Pandemic|pandemic]].<ref name="MargaretChanJune11">{{Cite news |url= https://www.who.int/mediacentre/news/statements/2009/h1n1_pandemic_phase6_20090611/en/index.html |title=World now at the start of 2009 influenza pandemic| vauthors = Chan M |date=11 June 2009|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=25 October 2009| archive-url= https://web.archive.org/web/20091022123810/http://www.who.int/mediacentre/news/statements/2009/h1n1_pandemic_phase6_20090611/en/index.html| archive-date= 22 October 2009 }}</ref>

Despite being informally called "swine flu", the H1N1 flu virus cannot be spread by eating pork products;<ref name="fluandyou"/><ref>{{Cite news |url=http://www.latimes.com/news/nation-and-world/la-na-pigs-flu5-2009dec05,0,6902061.story |title=Don't call it 'swine flu,' farmers implore | vauthors = Huffstutter PJ |date=5 December 2009 |access-date=5 December 2009 |newspaper={{wipe|Los Angeles Times}} |url-status=live |archive-url=https://web.archive.org/web/20091207163910/http://www.latimes.com/news/nation-and-world/la-na-pigs-flu5-2009dec05,0,6902061.story |archive-date=7 December 2009 }}</ref> similar to other influenza viruses, it is typically contracted by person to person transmission through respiratory droplets.<ref name="cdc.gov">{{cite web|url=https://www.cdc.gov/h1n1flu/recommendations.htm|title=Updated Interim Recommendations for the Use of Antiviral Medications in the Treatment and Prevention of Influenza for the 2009–2010 Season|date=7 December 2009|work=H1N1 Flu|publisher=Centers for Disease Control and Prevention (CDC) |access-date=10 December 2009| archive-url= https://web.archive.org/web/20091210205013/http://www.cdc.gov/h1n1flu/recommendations.htm| archive-date= 10 December 2009 | url-status= live}}</ref> Symptoms usually last 4–6 days.<ref>{{cite web|url=http://emedicine.medscape.com/article/1673658-overview|title=H1N1 Influenza (Swine Flu)| vauthors = Bronze MS |date=13 November 2009|work=eMedicine|publisher=Medscape|access-date=10 December 2009| archive-url= https://web.archive.org/web/20091227210136/http://emedicine.medscape.com/article/1673658-overview| archive-date= 27 December 2009 | url-status= live}}</ref> Antivirals ([[Chemistry:Oseltamivir|oseltamivir]] or [[Chemistry:Zanamivir|zanamivir]]) were recommended for those with more severe symptoms or those in an at-risk group.<ref name=CDCOct16AntiviralRecommendations/>

The pandemic began to taper off in November 2009,<ref name="WSJ-graph">{{cite news | url=https://www.wsj.com/articles/SB10001424052748703429304575095743102260012 | work=The Wall Street Journal | vauthors = Mckay B | title=The Flu Season That Fizzled | date=2 March 2010 | access-date=14 April 2011 | url-status=live | archive-url=https://web.archive.org/web/20150117223520/http://www.wsj.com/articles/SB10001424052748703429304575095743102260012 | archive-date=17 January 2015 | df=dmy-all }}</ref> and by May 2010, the number of cases was in steep decline.<ref name="WHOMay14update">{{Cite news|url=https://www.who.int/csr/don/2010_05_14/en/index.html|title=Pandemic (H1N1) 2009|date=14 May 2010|work=Disease Outbreak News|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=14 May 2010| archive-url= https://web.archive.org/web/20100518163428/http://www.who.int/csr/don/2010_05_14/en/index.html| archive-date= 18 May 2010 }}</ref><ref name="WHOIntensityMap">{{cite web|url=http://gamapserver.who.int/h1n1/qualitative_indicators/atlas.html?indicator=i2&date=Week%2017%20(26-Apr-2010%20:%2002-May-2010)|title=Global Intensity Map, Week 17 (26 April – 2 May 2010)|work=Global Update on 2009 H1N1|publisher=World Health Organization (WHO)|access-date=14 May 2010|archive-url=https://web.archive.org/web/20100519235058/http://gamapserver.who.int/h1n1/qualitative_indicators/atlas.html?indicator=i2&date=Week%2017%20(26-Apr-2010%20:%2002-May-2010)|archive-date=19 May 2010}}</ref><ref name="WHOSubtypeMap">{{cite web|url=https://www.who.int/csr/disease/swineflu/FluTransmissionZones_2010_05_14.png |title=Percentage of respiratory specimens that tested positive for influenza: Status as of week 17 (25 April – 1 May 2010) |publisher=World Health Organization (WHO) |access-date=14 May 2010 |archive-url=https://web.archive.org/web/20100519020752/http://www.who.int/csr/disease/swineflu/FluTransmissionZones_2010_05_14.png |archive-date=19 May 2010 |url-status=live }}</ref><ref name="CDCInternational">{{cite web|url=https://www.cdc.gov/h1n1flu/updates/international/|title=2009 H1N1 Flu: International Situation Update|date=7 May 2010|work=H1N1 Flu|publisher=Centers for Disease Control and Prevention (CDC) |access-date=14 May 2010| archive-url= https://web.archive.org/web/20100505140951/http://www.cdc.gov/h1n1flu/updates/international/| archive-date= 5 May 2010 | url-status= live}}</ref> On 10 August 2010, the Director-General of the WHO, Margaret Chan, announced the end of the H1N1 pandemic<ref name="WHO declares H1N1 Pandemic over">{{cite web|url=http://www.euro.who.int/en/what-we-do/health-topics/communicable-diseases/influenza/news2/news/2010/08/who-director-general-declares-h1n1-pandemic-over |title=WHO Director-General declares H1N1 pandemic over |publisher=World Health Organization (WHO) |date=10 August 2010 |access-date=24 May 2011| archive-url= https://web.archive.org/web/20110525050814/http://www.euro.who.int/en/what-we-do/health-topics/communicable-diseases/influenza/news2/news/2010/08/who-director-general-declares-h1n1-pandemic-over| archive-date= 25 May 2011 | url-status= live}}</ref> and announced that the H1N1 influenza event had moved into the post-pandemic period.<ref>{{cite web|url=https://www.who.int/csr/don/2010_09_10/en/index.html |title=Influenza updates |publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |date=10 September 2010 |access-date=24 May 2011| archive-url= https://web.archive.org/web/20110523201451/http://www.who.int/csr/don/2010_09_10/en/index.html| archive-date= 23 May 2011 }}</ref> According to WHO statistics (as of July 2010), the virus had killed more than 18,000 people since it appeared in April 2009; however, they state that the total mortality (including deaths unconfirmed or unreported) from the H1N1 strain is "unquestionably higher".<ref name="WHOMay14update"/><ref>{{Cite news|url=http://www.redorbit.com/news/health/1893907/h1n1_still_a_pandemic_says_who/|title=H1N1 Still A Pandemic, Says WHO|publisher=redOrbit|access-date=10 August 2010|url-status=live|archive-url=https://web.archive.org/web/20120929110039/http://www.redorbit.com/news/health/1893907/h1n1_still_a_pandemic_says_who/|archive-date=29 September 2012}}</ref> Critics claimed the WHO had exaggerated the danger, spreading "fear and confusion" rather than "immediate information".<ref>{{Cite news|title=WHO admits errors in handling flu pandemic: Agency accused of overplaying danger of the virus as it swept the globe|url=http://www.msnbc.msn.com/id/36421914/ |publisher=MSNBC.com|date=12 April 2010|access-date=14 April 2011 |archive-url=https://web.archive.org/web/20100420235803/http://www.msnbc.msn.com/id/36421914/|archive-date=20 April 2010}}</ref> The WHO began an investigation to determine<ref>{{Cite news|url=https://www.reuters.com/article/idUSTRE5BL2ZT20100112 |title=WHO to review its handling of H1N1 flu pandemic |work=Reuters | vauthors = Lynn J |date=12 January 2010 |access-date=14 April 2011 |archive-url=https://web.archive.org/web/20101124224735/https://www.reuters.com/article/idUSTRE5BL2ZT20100112 |archive-date=24 November 2010 |url-status=live }}</ref> whether it had "frightened people unnecessarily".<ref name="UKTimes-waste">{{Cite news|url=http://www.timesonline.co.uk/tol/news/world/article7104253.ece|title=Swine flu pandemic declared by WHO was 'a waste of money'|publisher=The Times (UK)|date=22 April 2010|location=London|access-date=14 April 2011 |url-status=live|archive-url=https://web.archive.org/web/20100531094130/http://www.timesonline.co.uk/tol/news/world/article7104253.ece|archive-date=31 May 2010}}</ref> A flu follow-up study done in September 2010, found that "the risk of most serious complications was not elevated in adults or children."<ref name="bloomberg.com">{{cite web | vauthors = Randall T |url=https://www.bloomberg.com/news/2010-09-07/swine-flu-in-children-is-no-more-severe-than-seasonal-virus-study-finds.html |title=Swine Flu In Children Found No More Severe Than Seasonal Virus |publisher=Bloomberg |date=7 September 2010 |access-date=30 January 2011 |url-status=live |archive-url=https://web.archive.org/web/20100910101552/http://www.bloomberg.com/news/2010-09-07/swine-flu-in-children-is-no-more-severe-than-seasonal-virus-study-finds.html |archive-date=10 September 2010 }}</ref> In a 5{{nbsp}}August 2011 ''PLOS ONE'' article, researchers estimated that the 2009 H1N1 global infection rate was 11% to 21%, lower than what was previously expected.<ref>{{cite news | vauthors = Roos R |title=Study puts global 2009 H1N1 infection rate at 11% to 21% |url= http://www.cidrap.umn.edu/cidrap/content/influenza/swineflu/news/aug0811serologic.html |newspaper=CIDRAP|date=8 August 2011|access-date=10 August 2011 |url-status=live|archive-url= https://web.archive.org/web/20120819145419/http://www.cidrap.umn.edu/cidrap/content/influenza/swineflu/news/aug0811serologic.html |archive-date=19 August 2012}}</ref> However, by 2012, research showed that as many as 579,000 people could have been killed by the disease, as only those fatalities confirmed by laboratory testing were included in the original number, and meant that many without access to health facilities went uncounted. The majority of these deaths occurred in Africa and Southeast Asia. Experts, including the WHO, have agreed that an estimated 284,500 people were killed by the disease, much higher than the initial death toll.<ref>{{cite journal | vauthors = Dawood FS, Iuliano AD, Reed C, Meltzer MI, Shay DK, Cheng PY, Bandaranayake D, Breiman RF, Brooks WA, Buchy P, Feikin DR, Fowler KB, Gordon A, Hien NT, Horby P, Huang QS, Katz MA, Krishnan A, Lal R, Montgomery JM, Mølbak K, Pebody R, Presanis AM, Razuri H, Steens A, Tinoco YO, Wallinga J, Yu H, Vong S, Bresee J, Widdowson MA | display-authors = 6 | title = Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: a modelling study | journal = The Lancet. Infectious Diseases | volume = 12 | issue = 9 | pages = 687–95 | date = September 2012 | pmid = 22738893 | doi = 10.1016/S1473-3099(12)70121-4 | url = https://zenodo.org/record/1260250 | access-date = 6 September 2020 | archive-date = 5 March 2023 | archive-url = https://web.archive.org/web/20230305165544/https://zenodo.org/record/1260250 | url-status = live }}</ref><ref>{{cite web|url=https://www.cdc.gov/flu/spotlights/pandemic-global-estimates.htm|title=First Global Estimates of 2009 H1N1 Pandemic Mortality Released by CDC-Led Collaboration|date=25 June 2012|publisher=Centers for Disease Control and Prevention (CDC)|access-date=3 July 2012|url-status=live|archive-url=https://web.archive.org/web/20120701003130/http://www.cdc.gov/flu/spotlights/pandemic-global-estimates.htm|archive-date=1 July 2012}}</ref>

==Signs and symptoms==


The symptoms of H1N1 flu are similar to those of other [[Medicine:Influenza|influenza]]s, and may include fever, cough (typically a "dry cough"), headache, dizziness, sneezing, [[Medicine:Myalgia|muscle]] or [[Medicine:Arthralgia|joint]] pain, sore throat, chills, fatigue, and runny nose. [[Medicine:Diarrhea|Diarrhea]], vomiting, and [[Biology:Neurology|neurological]] problems have also been reported in some cases.<ref name="CDC090724">{{cite web|url=https://www.cdc.gov/media/transcripts/2009/t090724.htm |title=CDC Briefing on Investigation of Human Cases of H1N1 Flu |date=24 July 2009 |publisher=Centers for Disease Control and Prevention (CDC) |access-date=28 July 2009 |archive-url=https://web.archive.org/web/20090829202445/http://www.cdc.gov/media/transcripts/2009/t090724.htm |archive-date= 29 August 2009 |url-status=live }}</ref><ref name="CDCgeninfo">{{cite web|url=https://www.cdc.gov/h1n1flu/guidance_homecare.htm|title=Interim Guidance for 2009 H1N1 Flu (Swine Flu): Taking Care of a Sick Person in Your Home|date=5 August 2009|publisher=Centers for Disease Control and Prevention (CDC) |access-date=1 November 2009| archive-url= https://web.archive.org/web/20091103194946/http://www.cdc.gov/h1n1flu/guidance_homecare.htm| archive-date= 3 November 2009 | url-status= live}}</ref> People at higher risk of serious complications include people over 65, children younger than 5, children with [[Medicine:Neurodevelopmental disorder|neurodevelopmental conditions]], pregnant women (especially during the third trimester),<ref name="WHObriefingnoteOct16"/><ref name="AndrePicardNov1">{{Cite news|url=https://www.theglobeandmail.com/life/health/h1n1-swine-flu/updated-reader-questions-on-h1n1-answered/article1329448/|title=Reader questions on H1N1 answered| vauthors = Picard A |date=1 November 2009| newspaper=The Globe and Mail|access-date=2 November 2009|location=Toronto, Canada| archive-url= https://web.archive.org/web/20091104040801/http://www.theglobeandmail.com/life/health/h1n1-swine-flu/updated-reader-questions-on-h1n1-answered/article1329448/| archive-date= 4 November 2009 }}</ref> and people of any age with underlying medical conditions, such as asthma, diabetes, obesity, heart disease, or a weakened immune system (e.g., taking immunosuppressive medications or infected with HIV).<ref name="NYT5-27">{{Cite news| vauthors = Hartocollis A |url=https://www.nytimes.com/2009/05/28/health/policy/28flu.html |title='Underlying conditions' may add to flu worries | newspaper={{wipe|The New York Times}} |date=27 May 2009|access-date=26 September 2009| archive-url= https://web.archive.org/web/20130425070939/http://www.nytimes.com/2009/05/28/health/policy/28flu.html| archive-date=25 April 2013| url-status= live}}</ref> More than 70% of hospitalizations in the U.S. have been people with such underlying conditions, according to the CDC.<ref>{{Cite news|url=https://www.wsj.com/articles/SB124502116646013713 |title=Flu Pandemic Spurs Queries About Vaccine | vauthors = Whalen J |date=15 June 2009 |newspaper={{wipe|The Wall Street Journal}} |access-date=31 August 2009 |archive-url=https://web.archive.org/web/20091007164158/http://online.wsj.com/article/SB124502116646013713.html |archive-date= 7 October 2009 |url-status=live }}</ref>

In September 2009, the CDC reported that the H1N1 flu "seems to be taking a heavier toll among chronically ill children than the seasonal flu usually does".<ref name=DeniseGradySept3>{{Cite news|url=https://www.nytimes.com/2009/09/04/health/research/04flu-001.html|title=Report Finds Swine Flu Has Killed 36 Children| vauthors = Grady D |date=3 September 2009|newspaper={{wipe|The New York Times}}|access-date=17 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007164316/http://www.nytimes.com/2009/09/04/health/research/04flu-001.html|archive-date=7 October 2009}}</ref> Through 8{{nbsp}}August 2009, the CDC had received 36 reports of pediatric deaths with associated influenza symptoms and laboratory-confirmed pandemic H1N1 from state and local health authorities within the United States, with 22 of these children having neurodevelopmental conditions such as [[Medicine:Cerebral palsy|cerebral palsy]], [[Medicine:Muscular dystrophy|muscular dystrophy]], or developmental delays.<ref name="CDCSept4">{{Cite news|url=https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5834a1.htm|title=Surveillance for Pediatric Deaths Associated with 2009 Pandemic Influenza A (H1N1) Virus Infection – United States, April–August 2009|date=4 September 2009|work=Morbidity and Mortality Weekly Report|publisher=Centers for Disease Control and Prevention (CDC) |access-date=2 December 2009| archive-url= https://web.archive.org/web/20091126022451/http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5834a1.htm| archive-date= 26 November 2009 | url-status= live}}</ref> "Children with nerve and muscle problems may be at especially high risk for complications because they cannot cough hard enough to clear their airways".<ref name= DeniseGradySept3/> From 26 April 2009, to 13 February 2010, the CDC had received reports of the deaths of 277 children with laboratory-confirmed 2009 influenza A (H1N1) within the United States.<ref>{{cite web|url=https://www.cdc.gov/flu/weekly/|title=2009–2010 Influenza Season Week 6 ending 13 February 2010|publisher=CDC|work=Fluview: A Weekly Influenza Surveillance Report Prepared by the Influenza Division|url-status=live|archive-url=https://web.archive.org/web/20110219120154/http://www.cdc.gov/flu/weekly/|archive-date=19 February 2011|date=15 February 2019}}</ref>

===Severe cases===

The [[Organization:World Health Organization|World Health Organization]] reports that the clinical picture in severe cases is strikingly different from the disease pattern seen during epidemics of seasonal influenza. While people with certain underlying medical conditions are known to be at increased risk, many severe cases occur in previously healthy people. In severe cases, patients generally begin to deteriorate around three to five days after symptom onset. Deterioration is rapid, with many patients progressing to respiratory failure within 24 hours, requiring immediate admission to an [[Medicine:Intensive care unit|intensive care unit]]. Upon admission, most patients need immediate respiratory support with [[Medicine:Mechanical ventilation|mechanical ventilation]].<ref>{{Cite news|url=https://www.who.int/csr/disease/swineflu/notes/h1n1_clinical_features_20091016/en/index.html|title=Clinical features of severe cases of pandemic influenza|date=16 October 2009|work=Pandemic (H1N1) 2009 briefing note 13|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=17 January 2010|location=Geneva, Switzerland| archive-url= https://web.archive.org/web/20100119214956/http://www.who.int/csr/disease/swineflu/notes/h1n1_clinical_features_20091016/en/index.html| archive-date= 19 January 2010 }}</ref>

===Complications===

Most complications have occurred among previously unhealthy individuals, with obesity and [[Medicine:Respiratory disease|respiratory disease]] as the strongest risk factors. Pulmonary complications are common. Primary influenza [[Medicine:Pneumonia|pneumonia]] occurs most commonly in adults and may progress rapidly to acute lung injury requiring [[Medicine:Mechanical ventilation|mechanical ventilation]]. Secondary [[Medicine:Bacterial pneumonia|bacterial infection]] is more common in children. ''[[Medicine:Staphylococcus aureus|Staphylococcus aureus]]'', including methicillin-resistant strains, is an important cause of secondary bacterial pneumonia with a high mortality rate; ''[[Biology:Streptococcus pneumoniae|Streptococcus pneumoniae]]'' is the second most important cause of secondary bacterial pneumonia for children and primary for adults. Neuromuscular and cardiac complications are unusual but may occur.<ref>{{cite journal | vauthors = Rothberg MB, Haessler SD | title = Complications of seasonal and pandemic influenza | journal = Critical Care Medicine | volume = 38 | issue = 4 Suppl | pages = e91–97 | date = April 2010 | pmid = 19935413 | doi = 10.1097/CCM.0b013e3181c92eeb | s2cid = 43882788 | doi-access = free }}</ref>

A United Kingdom investigation of risk factors for hospitalisation and poor outcome with pandemic A/H1N1 influenza looked at 631 patients from 55 hospitals admitted with confirmed infection from May through September 2009. 13% were admitted to a high dependency or intensive care unit and 5% died; 36% were aged <16 years and 5% were aged ≥65 years. Non-white and pregnant patients were over-represented. 45% of patients had at least one underlying condition, mainly [[Medicine:Asthma|asthma]], and 13% received antiviral drugs before admission. Of 349 with documented chest x-rays on admission, 29% had evidence of [[Medicine:Pneumonia|pneumonia]], but bacterial co-infection was uncommon. Multivariate analyses showed that physician-recorded obesity on admission and pulmonary conditions other than asthma or [[Medicine:Chronic obstructive pulmonary disease|chronic obstructive pulmonary disease]] (COPD) were associated with a severe outcome, as were radiologically confirmed pneumonia and a raised [[Chemistry:C-reactive protein|C-reactive protein]] (CRP) level (≥{{Nowrap|100 mg/L)}}. 59% of all in-hospital deaths occurred in previously healthy people.<ref>{{cite journal | vauthors = Nguyen-Van-Tam JS, Openshaw PJ, Hashim A, Gadd EM, Lim WS, Semple MG, Read RC, Taylor BL, Brett SJ, McMenamin J, Enstone JE, Armstrong C, Nicholson KG | display-authors = 6 | title = Risk factors for hospitalisation and poor outcome with pandemic A/H1N1 influenza: United Kingdom first wave (May–September 2009) | journal = Thorax | volume = 65 | issue = 7 | pages = 645–51 | date = July 2010 | pmid = 20627925 | pmc = 2921287 | doi = 10.1136/thx.2010.135210 }}</ref>

[[Medicine:Fulminant|Fulminant]] (sudden-onset) [[Medicine:Myocarditis|myocarditis]] has been linked to infection with H1N1, with at least four cases of myocarditis confirmed in patients also infected with A/H1N1. Three out of the four cases of H1N1-associated myocarditis were classified as fulminant, and one of the patients died.<ref>{{cite journal | vauthors = Bratincsák A, El-Said HG, Bradley JS, Shayan K, Grossfeld PD, Cannavino CR | title = Fulminant myocarditis associated with pandemic H1N1 influenza A virus in children | journal = Journal of the American College of Cardiology | volume = 55 | issue = 9 | pages = 928–29 | date = March 2010 | pmid = 20153131 | doi = 10.1016/j.jacc.2010.01.004| doi-access = free }} (primary source)
*{{lay source |template=cite web |url=http://www.modernmedicine.com/modernmedicine/Modern+Medicine+Now/Myocarditis-Linked-to-Pandemic-H1N1-Flu-in-Childre/ArticleNewsFeed/Article/detail/656746?contextCategoryId=40137 |title=Myocarditis Linked to Pandemic H1N1 Flu in Children |website=Modern Medicine |date=2010-02-11}}</ref>
Also, there appears to be a link between severe A/H1N1 influenza infection and [[Medicine:Pulmonary embolism|pulmonary embolism]]. In one report, five out of 14 patients admitted to the intensive care unit with severe A/H1N1 infection were found to have pulmonary emboli.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2009/10/091014111549.htm |title=Those With Severe H1N1 At Risk For Pulmonary Emboli, Researchers Find |publisher=ScienceDaily |date=18 October 2009 |access-date=3 April 2010 |url-status=live |archive-url=https://web.archive.org/web/20100307163910/http://www.sciencedaily.com/releases/2009/10/091014111549.htm |archive-date=7 March 2010 }}</ref>

An article published in ''JAMA'' in September 2010<ref>{{cite journal | vauthors = Belongia EA, Irving SA, Waring SC, Coleman LA, Meece JK, Vandermause M, Lindstrom S, Kempf D, Shay DK | display-authors = 6 | title = Clinical characteristics and 30-day outcomes for influenza A 2009 (H1N1), 2008–2009 (H1N1), and 2007–2008 (H3N2) infections | journal = JAMA | volume = 304 | issue = 10 | pages = 1091–98 | date = September 2010 | pmid = 20823435 | doi = 10.1001/jama.2010.1277 | doi-access = free }}</ref> challenged previous reports and stated that children infected in the 2009 flu pandemic were no more likely to be hospitalised with complications or get pneumonia than those who catch seasonal strains. Researchers found that about 1.5% of children with the H1N1 swine flu strain were hospitalised within 30 days, compared with 3.7% of those sick with a seasonal strain of H1N1 and 3.1% with an H3N2 virus.<ref name="bloomberg.com"/>

==Diagnosis==

Confirmed diagnosis of pandemic H1N1 flu requires testing of a [[Nasopharyngeal swab|nasopharyngeal]], nasal, or oropharyngeal tissue swab from the patient.<ref name="specimen">{{cite web|url=https://www.cdc.gov/h1n1flu/specimencollection.htm|title=Interim Guidance on Specimen Collection, Processing, and Testing for Patients with Suspected Novel Influenza A (H1N1) Virus Infection |date=13 May 2009|publisher=Centers for Disease Control and Prevention (CDC) |access-date=23 November 2009| archive-url= https://web.archive.org/web/20091126020551/http://www.cdc.gov/h1n1flu/specimencollection.htm| archive-date= 26 November 2009 | url-status= live}}</ref> [[Biology:Real-time polymerase chain reaction|Real-time]] RT-PCR is the recommended test as others are unable to differentiate between pandemic H1N1 and regular seasonal flu.<ref name="specimen"/> However, most people with flu symptoms do not need a test for pandemic H1N1 flu specifically, because the test results usually do not affect the recommended course of treatment.<ref name="diagpub">{{cite web|url=https://www.cdc.gov/h1n1flu/diagnostic_testing_public_qa.htm|title=Influenza Diagnostic Testing During the 2009–2010 Flu Season|date=29 September 2009|work=H1N1 Flu|publisher = Centers for Disease Control and Prevention (CDC) |access-date=23 November 2009| archive-url= https://web.archive.org/web/20091126004503/http://www.cdc.gov/h1n1flu/diagnostic_testing_public_qa.htm| archive-date= 26 November 2009 | url-status= live}}</ref> The U.S. CDC recommend testing only for people who are hospitalized with suspected flu, pregnant women, and people with weakened immune systems.<ref name="diagpub"/> For the mere diagnosis of influenza and not pandemic H1N1 flu specifically, more widely available tests include [[Biology:Rapid influenza diagnostic test|rapid influenza diagnostic test]]s (RIDT), which yield results in about 30 minutes, and [[Direct fluorescent antibody|direct]] and indirect [[Biology:Immunofluorescence|immunofluorescence]] assays ([[Direct fluorescent antibody|DFA]] and IFA), which take 2–4 hours.<ref name="diagnostic">{{cite web|url=https://www.cdc.gov/h1n1flu/guidance/diagnostic_tests.htm|title=Interim Recommendations for Clinical Use of Influenza Diagnostic Tests During the 2009–10 Influenza Season|date=29 September 2009|work=H1N1 Flu|publisher = Centers for Disease Control and Prevention (CDC) |access-date=23 November 2009| archive-url= https://web.archive.org/web/20091126022419/http://www.cdc.gov/h1n1flu/guidance/diagnostic_tests.htm| archive-date= 26 November 2009 | url-status= live}}</ref> Due to the high rate of RIDT false negatives, the CDC advises that patients with illnesses compatible with novel influenza A (H1N1) virus infection but with negative RIDT results should be treated empirically based on the level of clinical suspicion, underlying medical conditions, severity of illness, and risk for complications, and if a more definitive determination of infection with influenza virus is required, testing with rRT-PCR or virus isolation should be performed.<ref>{{Cite news|url=https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5830a2.htm|title=Evaluation of Rapid Influenza Diagnostic Tests for Detection of Novel Influenza A (H1N1) Virus – United States, 2009|date=7 August 2009|work=Morbidity and Mortality Weekly Report|publisher = Centers for Disease Control and Prevention (CDC) |access-date=5 December 2009| archive-url= https://web.archive.org/web/20091217043745/http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5830a2.htm| archive-date= 17 December 2009 | url-status= live}}</ref> The use of RIDTs has been questioned by researcher Paul Schreckenberger of the Loyola University Health System, who suggests that rapid tests may actually pose a dangerous public health risk.<ref>{{Cite news|url=http://www.loyolamedicine.org/News/News_Releases/news_release_detail.cfm?var_news_release_id=973441074|title=Could Widely Used Rapid Influenza Tests Pose A Dangerous Public Health Risk?|date=17 November 2009|publisher=Loyola Medicine|access-date=4 December 2009|location=Maywood, Illinois, U.S.|url-status=live|archive-url=https://web.archive.org/web/20091207163917/http://www.loyolamedicine.org/News/News_Releases/news_release_detail.cfm?var_news_release_id=973441074|archive-date=7 December 2009}}</ref> Nikki Shindo of the WHO has expressed regret at reports of treatment being delayed by waiting for H1N1 test results and suggests, "[D]octors should not wait for the laboratory confirmation but make diagnosis based on clinical and epidemiological backgrounds and start treatment early."<ref name="NikkiShindoNov12"/>

On 22 June 2010, the CDC announced a new test called the "CDC Influenza 2009 A (H1N1)pdm Real-Time RT-PCR Panel (IVD)". It uses a molecular biology technique to detect influenza A viruses and specifically the 2009 H1N1 virus. The new test will replace the previous real-time RT-PCR diagnostic test used during the 2009 H1N1 pandemic, which received an emergency use authorization from the U.S. [[Organization:Food and Drug Administration|Food and Drug Administration]] in April 2009. Tests results are available in four hours and are 96% accurate.<ref>{{cite press release |url=https://www.cdc.gov/media/pressrel/2010/r100622.htm |title=New CDC Test to Detect Human Infections with the 2009 H1N1 Influenza Virus Authorized for Use by FDA |location=Atlanta |publisher=Centers for Disease Control and Prevention (CDC) |date=22 June 2010 |access-date=25 August 2010| archive-url= https://web.archive.org/web/20100826205400/http://www.cdc.gov/media/pressrel/2010/r100622.htm| archive-date= 26 August 2010 | url-status= live}}</ref>

==Cause==


The virus was found to be a novel strain of influenza for which existing [[Biology:Vaccination|vaccines]] against seasonal flu provided little protection. A study at the U.S. Centers for Disease Control and Prevention published in May 2009 found that children had no preexisting immunity to the new strain but that adults, particularly those older than 60, had some degree of [[Medicine:Immunity (medical)|immunity]]. Children showed no [[Biology:Cross-reactivity|cross-reactive]] [[Biology:Antibody|antibody]] reaction to the new strain, adults aged 18 to 60 had 6–9%, and older adults 33%.<ref>{{cite web|url=http://www.cidrap.umn.edu/cidrap/content/influenza/swineflu/news/may2109serum-jw.html |title=Some immunity to novel H1N1 flu found in seniors | vauthors = Schnirring L |date=21 May 2009 |publisher=Center for Infectious Disease Research & Policy |access-date=26 September 2009| archive-url= https://web.archive.org/web/20090923225423/http://www.cidrap.umn.edu/cidrap/content/influenza/swineflu/news/may2109serum-jw.html| archive-date= 23 September 2009 | url-status= live}}</ref><ref name="MMWR">{{cite journal |author1=Centers for Disease Control Prevention (CDC) |date=24 April 2009 |title=Swine Influenza A (H1N1) Infection in Two Children – Southern California, March–April 2009 |url=http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5815a5.htm |url-status=live |journal=Morbidity and Mortality Weekly Report |publisher=CDC |volume=58 |issue=15 |pages=400–402 |pmid=19390508 |archive-url=https://web.archive.org/web/20090910141510/http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5815a5.htm |archive-date=10 September 2009 |access-date=25 April 2009}}</ref> While it has been thought that these findings suggest the partial immunity in older adults may be due to previous exposure to similar seasonal influenza viruses, a November 2009 study of a rural unvaccinated population in China found only a 0.3% cross-reactive antibody reaction to the H1N1 strain, suggesting that previous vaccinations for seasonal flu and not exposure may have resulted in the immunity found in the older U.S. population.<ref>{{cite journal | vauthors = Chen H, Wang Y, Liu W, Zhang J, Dong B, Fan X, de Jong MD, Farrar J, Riley S, Smith GJ, Guan Y | display-authors = 6 | title = Serologic survey of pandemic (H1N1) 2009 virus, Guangxi Province, China | journal = Emerging Infectious Diseases | volume = 15 | issue = 11 | pages = 1849–50 | date = November 2009 | pmid = 19891883 | pmc = 2857250 | doi = 10.3201/eid1511.090868 }}</ref>

Analyses of the genetic sequences of the first isolates, promptly shared on the [[Organization:GISAID|GISAID]] database according to Nature and WHO,<ref>{{cite journal | vauthors = Butler D | title = Swine flu goes global | journal = Nature | volume = 458 | issue = 7242 | pages = 1082–83 | date = April 2009 | pmid = 19407756 | doi = 10.1038/4581082a | doi-access = free }}</ref><ref>{{Cite web|url=https://www.who.int/csr/disease/swineflu/Gene_sequences_20090425.pdf|title=Viral gene sequences to assist update diagnostics for swine influenza A(H1N1)|date=15 April 2009|website=World Health Organization|access-date=6 September 2017|url-status=live|archive-url=https://web.archive.org/web/20170807182539/http://www.who.int/csr/disease/swineflu/Gene_sequences_20090425.pdf|archive-date=7 August 2017}}</ref> soon determined that the strain contains genes from five different flu viruses: North American swine influenza, North American avian influenza, human influenza, and two swine influenza viruses typically found in Asia and Europe. Further analysis has shown that several [[Biology:Protein|protein]]s of the virus are most similar to strains that cause mild symptoms in humans, leading virologist Wendy Barclay to suggest on 1{{nbsp}}May 2009, that the initial indications are that the virus was unlikely to cause severe symptoms for most people.<ref>{{Cite news|url=http://news.bbc.co.uk/2/hi/health/8028371.stm |title=What scientists know about swine flu |work=BBC News |author=Emma Wilkinson |date=1 May 2009|access-date=26 September 2009| archive-url= https://web.archive.org/web/20090929231136/http://news.bbc.co.uk/2/hi/health/8028371.stm| archive-date= 29 September 2009 | url-status= live}}</ref>

The virus was less lethal than previous pandemic strains and killed about 0.01–0.03% of those infected; the 1918 influenza was about one hundred times more lethal and had a case fatality rate of 2–3%.<ref>{{cite journal | vauthors = Donaldson LJ, Rutter PD, Ellis BM, Greaves FE, Mytton OT, Pebody RG, Yardley IE | title = Mortality from pandemic A/H1N1 2009 influenza in England: public health surveillance study | journal = BMJ | volume = 339 | pages = b5213 | date = December 2009 | pmid = 20007665 | pmc = 2791802 | doi = 10.1136/bmj.b5213 }}</ref> By 14 November 2009, the virus had infected one in six Americans with 200,000 hospitalisations and 10,000 deaths—as many hospitalizations and fewer deaths than in an average flu season overall, but with much higher risk for those under 50. With deaths of 1,100 children and 7,500 adults 18 to 64, these figures were deemed "much higher than in a usual flu season" during the pandemic.<ref>{{Cite news|url=http://www.latimes.com/news/nation-and-world/la-sci-swine-flu11-2009dec11,0,6351332.story |title=Swine flu has hit about one in six Americans, CDC says | vauthors = Maugh II TH |work=Los Angeles Times |date=11 December 2009 |access-date=13 December 2009 |archive-url=https://web.archive.org/web/20091213114305/http://www.latimes.com/news/nation-and-world/la-sci-swine-flu11-2009dec11%2C0%2C6351332.story |archive-date=13 December 2009 |url-status=live }}</ref>

In June 2010, scientists from Hong Kong reported discovery of a new swine flu virus: a hybrid of the pandemic H1N1 virus and viruses previously found in pigs. It was the first report of a reassortment of the pandemic virus, which in humans had been slow to evolve. Nancy Cox, head of the influenza division at the U.S. Centers for Disease Control and Prevention, has said, "This particular paper is extremely interesting because it demonstrates for the first time what we had worried about at the very onset of the pandemic, and that is that this particular virus, when introduced into pigs, could reassort with the resident viruses in pigs and we would have new gene constellations. And bingo, here we are." Pigs have been termed the mixing vessel of flu because they can be infected both by avian flu viruses, which rarely directly infect people, and by human viruses. When pigs become simultaneously infected with more than one virus, the viruses can swap genes, producing new variants which can pass to humans and sometimes spread amongst them.<ref>{{cite journal | vauthors = Vijaykrishna D, Poon LL, Zhu HC, Ma SK, Li OT, Cheung CL, Smith GJ, Peiris JS, Guan Y | display-authors = 6 | title = Reassortment of pandemic H1N1/2009 influenza A virus in swine | journal = Science | volume = 328 | issue = 5985 | pages = 1529 | date = June 2010 | pmid = 20558710 | pmc = 3569847 | doi = 10.1126/science.1189132 | bibcode = 2010Sci...328.1529V }}</ref> "Unlike the situation with birds and humans, we have a situation with pigs and humans where there's a two-way street of exchange of viruses. With pigs it's very much a two-way street."<ref>{{Cite news |url=https://www.ctvnews.ca/h1n1-swaps-genes-with-other-pig-viruses-scientists-1.523986 |title=H1N1 swaps genes with other pig viruses: scientists |date=18 June 2010 |publisher=CTV News |agency=The Canadian Press |access-date=14 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110727124022/http://edmonton.ctv.ca/servlet/an/local/CTVNews/20100618/swine-flu-virus-100618/20100618/?hub=EdmontonHome |archive-date=27 July 2011 }}</ref>

===Transmission===

Spread of the H1N1 virus is thought to occur in the same way that seasonal flu spreads. Flu viruses are spread mainly from person to person through coughing or sneezing by people with influenza. Sometimes people may become infected by touching something—such as a surface or object—with flu viruses on it and then touching their face.<ref name="fluandyou">{{cite web|url=https://www.cdc.gov/H1N1flu/qa.htm|title=2009 H1N1 Flu ("Swine Flu") and You|date=10 February 2010|publisher=Centers for Disease Control and Prevention (CDC) |access-date=26 February 2010| archive-url= https://web.archive.org/web/20100304031110/http://www.cdc.gov/h1n1flu/qa.htm| archive-date= 4 March 2010 | url-status= live}}</ref>

The [[Medicine:Basic reproduction number|basic reproduction number]] (the average number of other individuals whom each infected individual will infect, in a population which has no immunity to the disease) for the 2009 novel H1N1 is estimated to be 1.75.<ref>{{cite journal | vauthors = Balcan D, Hu H, Goncalves B, Bajardi P, Poletto C, Ramasco JJ, Paolotti D, Perra N, Tizzoni M, Van den Broeck W, Colizza V, Vespignani A | display-authors = 6 | title = Seasonal transmission potential and activity peaks of the new influenza A(H1N1): a Monte Carlo likelihood analysis based on human mobility | journal = BMC Medicine | volume = 7 | issue = 45 | pages = 45 | date = September 2009 | pmid = 19744314 | pmc = 2755471 | doi = 10.1186/1741-7015-7-45 | arxiv = 0909.2417 | bibcode = 2009arXiv0909.2417B | doi-access = free }}</ref> A December 2009 study found that the transmissibility of the H1N1 influenza virus in households is lower than that seen in past pandemics. Most transmissions occur soon before or after the onset of symptoms.<ref>{{cite journal | vauthors = Cauchemez S, Donnelly CA, Reed C, Ghani AC, Fraser C, Kent CK, Finelli L, Ferguson NM | display-authors = 6 | title = Household transmission of 2009 pandemic influenza A (H1N1) virus in the United States | journal = The New England Journal of Medicine | volume = 361 | issue = 27 | pages = 2619–27 | date = December 2009 | pmid = 20042753 | pmc = 3840270 | doi = 10.1056/NEJMoa0905498 |  author-link2 = Christl Donnelly }}</ref>

The H1N1 virus has been transmitted to animals, including swine, [[Biology:Turkey (bird)|turkeys]], [[Biology:Ferret|ferret]]s, household cats, at least one dog, and a [[Biology:Cheetah|cheetah]].<ref>{{Cite news|url=http://consults.blogs.nytimes.com/2009/11/05/can-pets-get-swine-flu/000 |title=Can Pets Get Swine Flu? | vauthors = Murray L |newspaper={{wipe|The New York Times}} |date=5 November 2009 |access-date=6 November 2009 |archive-url=https://web.archive.org/web/20091108173748/http://consults.blogs.nytimes.com/2009/11/05/can-pets-get-swine-flu/ |archive-date= 8 November 2009 |url-status=live }}</ref><ref>{{Cite news | url=http://well.blogs.nytimes.com/2009/11/05/the-cat-who-got-swine-flu/ | title=The Cat Who Got Swine Flu | vauthors = Parker-Pope T | newspaper={{wipe|The New York Times}} | date=5 November 2009 | access-date=6 November 2009| archive-url=https://web.archive.org/web/20091108150018/http://well.blogs.nytimes.com/2009/11/05/the-cat-who-got-swine-flu/| archive-date= 8 November 2009 | url-status= live}}</ref><ref>
{{Cite news |url=http://www.cbc.ca/health/story/2009/12/22/h1n1-us-dog-who.html |title=Pet dog recovers from H1N1 |newspaper=CBC News | date=22 December 2009|archive-url=https://web.archive.org/web/20091224082642/http://www.cbc.ca/health/story/2009/12/22/h1n1-us-dog-who.html |archive-date=24 December 2009}}</ref><ref>{{cite web |url=http://www.usda.gov/documents/FINAL_RESULTS_2009_PANDEMIC_H1N1_INFLUENZA_CHT.pdf |title=2009 Pandemic H1N1 Influenza Presumptive and Confirmed Results |date=4 December 2009 |publisher=United States Department of Agriculture |archive-url=https://web.archive.org/web/20091203233039/http://www.usda.gov/documents/FINAL_RESULTS_2009_PANDEMIC_H1N1_INFLUENZA_CHT.pdf|archive-date=3 December 2009}}</ref>

==Prevention==


Because the H1N1 vaccine was initially in short supply in the U.S., the CDC recommended that initial doses should go to priority groups such as pregnant women, people who live with or care for babies under six months old, children six months to four years old and health-care workers.<ref>{{cite web|url=https://www.cdc.gov/mmWR/PDF/rr/rr5810.pdf|title=Use of Influenza A (H1N1) 2009 Monovalent Vaccine: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009|work=Morbidity and Mortality Weekly Report|publisher=Centers for Disease Control and Prevention (CDC)|date=28 November 2009|access-date=2 November 2009| archive-url= https://web.archive.org/web/20091103202202/http://www.cdc.gov/mmwr/pdf/rr/rr5810.pdf| archive-date= 3 November 2009 | url-status= live}}</ref> In the UK, the NHS recommended vaccine priority go to people over six months old who were clinically at risk for seasonal flu, pregnant women and households of people with compromised immunity.<ref name="nhs-april">{{Cite news|url=http://www.nhs.uk/news/2009/04april/pages/swineflulatest.aspx|title=Swine flu latest from the NHS|date=25 September 2009|work=NHS Choices|publisher=NHS|access-date=28 September 2009|agency=NHS Knowledge Service| archive-url= https://web.archive.org/web/20091008013912/http://www.nhs.uk/News/2009/04April/Pages/swineflulatest.aspx| archive-date= 8 October 2009 | url-status= live}}</ref>

Although it was initially thought that two injections would be required, clinical trials showed that the new vaccine protected adults "with only one dose instead of two;" thus the limited vaccine supplies would go twice as far as had been predicted.<ref name="NYT9-12-09">{{Cite news|url=https://www.nytimes.com/2009/09/11/health/11vaccine.html|title=One Vaccine Shot Seen as Protective for Swine Flu| vauthors = McNeil Jr DG |date=10 September 2009|newspaper={{wipe|The New York Times}}|access-date=17 September 2009|url-status=live|archive-url= https://web.archive.org/web/20091007164052/http://www.nytimes.com/2009/09/11/health/11vaccine.html |archive-date=7 October 2009}}</ref><ref>{{cite web|url=https://www.who.int/csr/disease/swineflu/notes/briefing_20091030/en/index.html|title=Experts advise WHO on pandemic vaccine policies and strategies|date=30 October 2009|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO)|access-date=2 November 2009| archive-url= https://web.archive.org/web/20091102051824/http://www.who.int/csr/disease/swineflu/notes/briefing_20091030/en/index.html| archive-date= 2 November 2009 }}</ref> Health officials worldwide were also concerned because the virus was new and could easily mutate and become more virulent, even though most flu symptoms were mild and lasted only a few days without treatment. Officials also urged communities, businesses, and individuals to make contingency plans for possible school closures, multiple employee absences for illness, surges of patients in hospitals, and other effects of potentially widespread outbreaks.<ref name="WSJ-7-18-09">{{Cite news|url=https://www.wsj.com/articles/SB124787226825260533 |title=New Push in H1N1 Flu Fight Set for Start of School | vauthors = McKay B |date=18 July 2009 |newspaper={{wipe|The Wall Street Journal}} |access-date=31 August 2009 |archive-url=https://web.archive.org/web/20090922053424/http://online.wsj.com/article/SB124787226825260533.html?mod=googlenews_wsj |archive-date= 22 September 2009 |url-status=live }}</ref> Disaster response organizations such as Direct Relief helped by providing protective items to clinical workers to help them stay healthy throughout flu season.<ref>{{Cite web|url=https://www.directrelief.org/2009/11/478-community-clinics-49-states-receiving-needed-h1n1-protective-supplies/|title=478 Community Clinics in 49 States Receiving Needed H1N1 Protective Supplies|vauthors=Hutain J|date=21 November 2009|website=Direct Relief|access-date=25 April 2019|archive-date=25 April 2019|archive-url=https://web.archive.org/web/20190425140651/https://www.directrelief.org/2009/11/478-community-clinics-49-states-receiving-needed-h1n1-protective-supplies/|url-status=live}}</ref><ref>{{Cite web|url=http://www.nbcsandiego.com/news/health/Clinics-Swamped-with-Flu-Patients.html|title=Clinics Swamped with Flu Patients|vauthors=Stickney R|website=NBC 7 San Diego|date=4 May 2009|access-date=25 April 2019|archive-date=25 April 2019|archive-url=https://web.archive.org/web/20190425140254/https://www.nbcsandiego.com/news/health/Clinics-Swamped-with-Flu-Patients.html|url-status=live}}</ref>

In February 2010, the CDC's Advisory Committee on Immunization Practices voted for "universal" flu vaccination in the U.S. to include all people over six months of age. The 2010–2011 vaccine will protect against the 2009 H1N1 pandemic virus and two other flu viruses.<ref>{{cite web|url=https://www.cdc.gov/flu/about/qa/1011_vac_selection.htm |title=Vaccine Selection for the 2010–2011 Influenza Season |publisher=U.S. Centers for Disease Control and Prevention (CDC) |access-date=25 August 2010 |archive-url=https://web.archive.org/web/20101006224851/http://www.cdc.gov/flu/about/qa/1011_vac_selection.htm |archive-date=6 October 2010 }}</ref>

===Public health response===


On 27 April 2009, the [[Social:European Union|European Union]] health commissioner advised Europeans to postpone nonessential travel to the United States or Mexico. This followed the discovery of the first confirmed case in Spain.<ref name="Guardian 27/4">{{Cite news|url=https://www.theguardian.com/world/2009/apr/27/swine-flu-mexico|title=Europeans urged to avoid Mexico and US as swine flu death toll exceeds 100|date=27 April 2009|work=Guardian|access-date=27 April 2009 | location=London | vauthors = Nasaw D | archive-url= https://web.archive.org/web/20090430094402/http://www.guardian.co.uk/world/2009/apr/27/swine-flu-mexico| archive-date= 30 April 2009 | url-status= live}}</ref> On 6{{nbsp}}May 2009, the [[Organization:Public Health Agency of Canada|Public Health Agency of Canada]] announced that their National Microbiology Laboratory (NML) had mapped the genetic code of the swine flu virus, the first time that had been done.<ref>{{Cite news|url=http://www.iol.co.za/index.php?art_id=nw20090506191835578C369647&click_id=31&set_id=1 |title=H1N1 virus genome: 'This is a world first' |date=6 May 2009 |work=Independent |access-date=31 August 2009 |author=AFP |archive-url=https://web.archive.org/web/20091007163941/http://www.iol.co.za/index.php?art_id=nw20090506191835578C369647&click_id=31&set_id=1 |archive-date= 7 October 2009 }}</ref> In the U.K., the National Health Service launched a website, the National Pandemic Flu Service,<ref>{{cite web|url=https://www.pandemicflu.direct.gov.uk/ |archive-url=https://web.archive.org/web/20091007163952/https://www.pandemicflu.direct.gov.uk/ |archive-date=7 October 2009 |title=National Pandemic Flu Service |publisher=NHS, NHS Scotland, NHS Wales, DHSSPS |access-date=4 October 2009 }}</ref> allowing patients to self-assess and get an authorisation number for antiviral medication. The system was expected to reduce the burden on general practitioners.<ref name="nhs-april"/>

U.S. officials observed that six years of concern about H5N1 avian flu did much to prepare for the current H1N1 outbreak, noting that after H5N1 emerged in Asia, ultimately killing about 60% of the few hundred people infected over the years, many countries took steps to try to prevent any similar crisis from spreading further.<ref>{{Cite news|url=http://www.pe.com/localnews/rivcounty/stories/PE_News_Local_S_swineflu08.4301c9d.html |archive-url=https://web.archive.org/web/20100107164010/http://www.pe.com/localnews/rivcounty/stories/PE_News_Local_S_swineflu08.4301c9d.html |archive-date=7 January 2010 |title=Health officials evaluate response to swine flu |vauthors = Hines L |date=7 June 2009 |work=Riverside Press-Enterprise |access-date=17 September 2009 }}</ref> The CDC and other U.S. governmental agencies<ref>{{Cite news|url=https://www.washingtonpost.com/wp-dyn/content/article/2009/08/09/AR2009080902447.html|title=Preparing for Swine Flu's Return| vauthors = Stein R |date=10 August 2009|newspaper=The Washington Post|access-date=4 October 2009|url-status=live|archive-url=https://web.archive.org/web/20091007164155/http://www.washingtonpost.com/wp-dyn/content/article/2009/08/09/AR2009080902447.html|archive-date=7 October 2009}}</ref> used the summer lull to take stock of the United States response to H1N1 flu and attempt to patch any gaps in the public health safety net before flu season started in early autumn.<ref name="reuters20090604">{{Cite news|url=https://www.reuters.com/article/domesticNews/idUSN0423008520090604?sp=true|title=As swine flu wanes, U.S. preparing for second wave| vauthors = Steenhuysen J |date=4 June 2009|work=Reuters|access-date=17 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007164813/http://www.reuters.com/article/domesticNews/idUSN0423008520090604?sp=true|archive-date=7 October 2009}}</ref> Preparations included planning a second influenza vaccination program in addition to the one for seasonal flu, and improving coordination between federal, state, and local governments and private health providers.<ref name="reuters20090604"/> On 24 October 2009, U.S. President [[Biography:Barack Obama|Obama]] declared swine flu a national emergency, giving Secretary of Health and Human Services Kathleen Sebelius authority to grant waivers to requesting hospitals from usual federal requirements.<ref name="ObamaAuthorizesSebelius">{{Cite news|url=https://www.washingtonpost.com/wp-dyn/content/article/2009/10/24/AR2009102401061.html|title=President Obama declares H1N1 flu a national emergency| vauthors = Shear MD, Stein R |date=24 October 2009|newspaper=The Washington Post|access-date=25 October 2009|url-status=live|archive-url=https://web.archive.org/web/20091107164213/http://www.washingtonpost.com/wp-dyn/content/article/2009/10/24/AR2009102401061.html|archive-date=7 November 2009}}</ref>

===Vaccines===

{{Main|Medicine:2009 swine flu pandemic vaccine}}
[[File:A nurse vaccinates Barack Obama against H1N1.jpg|thumb|President [[Biography:Barack Obama|Barack Obama]] being vaccinated against H1N1 flu on 20 December 2009]]

By 19 November 2009, doses of vaccine had been administered in over 16 countries. A 2009 review by the U.S. National Institutes of Health (NIH) concluded that the 2009 H1N1 vaccine has a safety profile similar to that of the seasonal vaccine.

In 2011, a study from the US Flu Vaccine Effectiveness Network estimated the overall effectiveness of all pandemic H1N1 vaccines at 56%. A CDC study released 28 January 2013, estimated that the Pandemic H1N1 vaccine saved roughly 300 lives and prevented about a million illnesses in the US. The study concluded that had the vaccination program started two weeks earlier, close to 60% more cases could have been prevented. The study was based on an effectiveness in preventing cases, hospitalizations, and deaths of 62% for all subgroups except people over 65, for whom the effectiveness was estimated at 43%. The effectiveness was based on European and Asian studies and expert opinion. The delay in vaccine administration demonstrated the shortcomings of the world's capacity for vaccine-production, as well as problems with international distribution. Some manufacturers and wealthy countries had concerns regarding liability and regulations, as well as the logistics of transporting, storing, and administering vaccines to be donated to poorer countries.<ref>{{cite journal | vauthors = Fineberg HV | title = Pandemic preparedness and response—lessons from the H1N1 influenza of 2009 | journal = The New England Journal of Medicine | volume = 370 | issue = 14 | pages = 1335–42 | date = April 2014 | pmid = 24693893 | doi = 10.1056/NEJMra1208802 | doi-access = free }}</ref>

===Accusations of conflict of interest===

In January 2010, Wolfgang Wodarg, a German deputy who trained as a physician and chaired the health committee at the Council of Europe, claimed that major firms had organized a "campaign of panic" to put pressure on the [[Organization:World Health Organization|World Health Organization]] (WHO) to declare a "false pandemic" to sell vaccines. Wodarg said the WHO's "false pandemic" flu campaign is "one of the greatest medicine scandals of the century". He said that the "false pandemic" campaign began in May 2009 in [[Place:Mexico City|Mexico City]], when a hundred or so "normal" reported influenza cases were declared to be the beginning of a threatening new pandemic, although he said there was little scientific evidence for it. Nevertheless, he argued that the WHO, "in cooperation with some big pharmaceutical companies and their scientists, re-defined pandemics," removing the statement that "an enormous amount of people have contracted the illness or died" from its existing definition and replacing it by stating simply that there has to be a virus, spreading beyond borders and to which people have no immunity.<ref>{{Cite news|url=http://www.news.com.au/world/swine-flu/story-e6frfkyi-1225818388508|title=Swine flu 'a false pandemic' to sell vaccines, expert says|date=13 January 2010|publisher=News.com|access-date=13 January 2010 | vauthors = Pollard C | archive-url= https://web.archive.org/web/20100114013335/http://www.news.com.au/world/swine-flu/story-e6frfkyi-1225818388508| archive-date= 14 January 2010 }}</ref>

The WHO responded by stating that they take their duty to provide independent advice seriously and guarded against interference from outside interests. Announcing a review of the WHO's actions, spokeswoman Fadela Chaib stated: "Criticism is part of an outbreak cycle. We expect and indeed welcome criticism and the chance to discuss it".<ref>{{cite news|url=https://www.reuters.com/article/idUSTRE5BL2ZT20100112 |work=Reuters |title=WHO to review its handling of H1N1 flu pandemic |date=12 January 2010 |access-date=14 April 2011 |archive-url=https://web.archive.org/web/20101124224735/https://www.reuters.com/article/idUSTRE5BL2ZT20100112 |archive-date=24 November 2010 |url-status=live }}</ref><ref>Bloomberg {{cite web |url=https://www.bloomberg.com/apps/news?pid=20601091&sid=ahj0H_RH8U68 |title=WHO to Clarify H1N1 Data After False Pandemic Claim (Update1) |access-date=21 August 2017 |publisher= Bloomberg |url-status=live |archive-url=https://web.archive.org/web/20100307164337/http://www.bloomberg.com/apps/news?pid=20601091&sid=ahj0H_RH8U68 |archive-date=7 March 2010 }}</ref> The WHO also stated on their website that "The world is going through a real pandemic. The description of it as a fake is wrong and irresponsible".<ref>{{Cite news |last=Fumento |first=Michael |date=5 February 2010 |title=Why The WHO Faked A Pandemic |work={{wipe|Forbes}} |url=https://www.forbes.com/2010/02/05/world-health-organization-swine-flu-pandemic-opinions-contributors-michael-fumento.html |access-date=21 February 2021 |archive-url=https://web.archive.org/web/20100208182828/https://www.forbes.com/2010/02/05/world-health-organization-swine-flu-pandemic-opinions-contributors-michael-fumento.html |archive-date=8 February 2010}}</ref> In March 2010, the Council of Europe launched an enquiry into "the influence of the pharmaceutical companies on the global swine flu campaign", and a preliminary report was in preparation.<ref>{{cite web| vauthors = Flynn P |url=http://assembly.coe.int/CommitteeDocs/2010/20100329_MemorandumPandemie_E.pdf|title=The handling of the H1N1 pandemic: more transparency needed|publisher=memorandum, Social, Health and Family Affairs Committee, The Council of Europe|date=23 March 2010|url-status=live|archive-url=https://web.archive.org/web/20110223021733/http://assembly.coe.int/CommitteeDocs/2010/20100329_MemorandumPandemie_E.pdf|archive-date=23 February 2011}}</ref>

On 12 April 2010, Keiji Fukuda, the WHO's top influenza expert, stated that the system leading to the declaration of a pandemic led to confusion about H1N1 circulating around the world and he expressed concern that there was a failure to communicate in regard to uncertainties about the new virus, which turned out to be not as deadly as feared. WHO Director-General Margaret Chan appointed 29 flu experts from outside the organization to conduct a review of WHO's handling of the H1N1 flu pandemic. She told them, "We want a frank, critical, transparent, credible and independent review of our performance."<ref name=letter>{{cite web|url=https://www.who.int/mediacentre/news/statements/2010/letter_bmj_20100608/en/index.html |publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |author=Chan M |date=8 June 2010 |title=WHO Director-General's letter to BMJ editors|access-date=30 January 2011| archive-url= https://web.archive.org/web/20110130000141/http://www.who.int/mediacentre/news/statements/2010/letter_bmj_20100608/en/index.html| archive-date= 30 January 2011 }}</ref>

In June 2010, Fiona Godlee, editor-in-chief of the ''BMJ'', published an editorial which criticised the WHO, saying that an investigation had disclosed that some of the experts advising WHO on the pandemic had financial ties with drug companies which were producing antivirals and vaccines.<ref>{{cite journal | vauthors = Godlee F | title = Conflicts of interest and pandemic flu | journal = BMJ | volume = 340 | pages = c2947 | date = June 2010 | pmid = 20525680 | doi = 10.1136/bmj.c2947 | s2cid = 323055 | url = http://www.bmj.com/content/340/bmj.c2947.full#ref-1 | url-status = live | archive-url = https://web.archive.org/web/20101221213507/http://www.bmj.com/content/340/bmj.c2947.full | archive-date = 21 December 2010 }}</ref> Margaret Chan, Director-General of the WHO, replied stating, "Without question, the ''BMJ'' feature and editorial will leave many readers with the impression that WHO's decision to declare a pandemic was at least partially influenced by a desire to boost the profits of the pharmaceutical industry. The bottom line, however, is that decisions to raise the level of pandemic alert were based on clearly defined virological and epidemiological criteria. It is hard to bend these criteria, no matter what the motive".<ref name=letter/>

===Infection control===
====Travel precautions====

[[File:ChineseFluInspectors.JPG|thumb|Flu inspection on a flight arriving in China]]
[[File:Airport Thermographic Camera.jpg|thumb|Thermal imaging camera and screen, photographed in an airport terminal in Greece. Thermal imaging can detect elevated body temperature, one of the signs of swine flu.]]

On 7 May 2009, the WHO stated that containment was not feasible and that countries should focus on mitigating the effect of the virus. They did not recommend closing borders or restricting travel.<ref name="WHO Travel">{{cite web|url=https://www.who.int/csr/disease/swineflu/frequently_asked_questions/travel/en/index.html|title=Influenza A(H1N1) – Travel|date=7 May 2009|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO)|access-date=17 June 2009| archive-url= https://web.archive.org/web/20090618104753/http://www.who.int/csr/disease/swineflu/frequently_asked_questions/travel/en/index.html| archive-date= 18 June 2009 }}</ref> On 26 April 2009, the Chinese government announced that visitors returning from flu-affected areas who experienced flu-like symptoms within two weeks would be quarantined.<ref name="reuters">{{Cite news|url=https://www.reuters.com/article/europeCrisis/idUST344175|title=Asia moves to ward off new flu virus|work=Reuters|date=9 February 2009|access-date=26 April 2009| archive-url= https://web.archive.org/web/20090430041910/https://www.reuters.com/article/europeCrisis/idUST344175| archive-date= 30 April 2009 | url-status= live}}</ref>

U.S. airlines had made no major changes as of the beginning of June 2009, but continued standing practices which include looking for passengers with symptoms of flu, [[Medicine:Measles|measles]] or other infections, and relying on in-flight [[Physics:Air filter|air filter]]s to ensure that aircraft were sanitised.<ref name="airlines"/> Masks were not generally provided by airlines and the CDC did not recommend that airline crews wear them.<ref name="airlines"/> Some non-U.S. airlines, mostly Asian, including Singapore Airlines, [[Company:China Eastern Airlines|China Eastern Airlines]], [[Company:China Southern Airlines|China Southern Airlines]], Cathay Pacific and Aeromexico, took measures such as stepping up cabin cleaning, installing state-of-the-art air filters and allowing in-flight staff to wear face masks.<ref name="airlines">{{Cite news|url=https://www.reuters.com/article/asiaCrisis/idUSN27536654|title=Global airlines move to reduce infection risks| vauthors = Jacobs K |date=3 June 2009|work=Reuters|access-date=17 September 2009|url-status=live|archive-url= https://web.archive.org/web/20091007164721/http://www.reuters.com/article/asiaCrisis/idUSN27536654 |archive-date=7 October 2009}}</ref>

According to studies conducted in Australia and Japan, screening individuals for influenza symptoms at airports during the 2009 H1N1 outbreak was not an effective method of infection control.<ref>{{cite journal | vauthors = Gunaratnam PJ, Tobin S, Seale H, Marich A, McAnulty J | title = Airport arrivals screening during pandemic (H1N1) 2009 influenza in New South Wales, Australia | journal = The Medical Journal of Australia | volume = 200 | issue = 5 | pages = 290–92 | date = March 2014 | pmid = 24641156 | doi = 10.5694/mja13.10832 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Nishiura H, Kamiya K | title = Fever screening during the influenza (H1N1-2009) pandemic at Narita International Airport, Japan | journal = BMC Infectious Diseases | volume = 11 | pages = 111 | date = May 2011 | pmid = 21539735 | pmc = 3096599 | doi = 10.1186/1471-2334-11-111 | doi-access = free }}</ref>

====Schools====

U.S. government officials were especially concerned about schools because the H1N1 flu virus appeared to disproportionately affect young and school-age people, between six months and 24 years of age.<ref>{{cite web|url=http://www.businesswire.com/portal/site/google/?ndmViewId=news_view&newsId=20090820006035&newsLang=en|title=New CDC H1N1 Guidance for Colleges, Universities, and Institutions of Higher Education|date=20 August 2009|work=Business Wire|access-date=17 September 2009|archive-date=7 October 2009|archive-url=https://web.archive.org/web/20091007163952/http://www.businesswire.com/portal/site/google/?ndmViewId=news_view&newsId=20090820006035&newsLang=en|url-status=live}}</ref>
The H1N1 outbreak led to numerous precautionary school closures in some areas. Rather than closing schools, the CDC recommended that students and school workers with flu symptoms should stay home for either seven days total, or until 24 hours after symptoms subsided, whichever was longer.<ref name=24hours>{{Cite news|url=http://www.chron.com/disp/story.mpl/health/6557550.html |title=Schools revamp swine flu plans for fall | vauthors = George C |date=1 August 2009 |newspaper=Houston Chronicle |access-date=31 August 2009 |archive-url=https://web.archive.org/web/20091007163939/http://www.chron.com/disp/story.mpl/health/6557550.html |archive-date= 7 October 2009 |url-status=live }}</ref> The CDC also recommended that colleges should consider suspending fall 2009 classes if the virus began to cause severe illness in a significantly larger share of students than the previous spring. They also urged schools to suspend rules, such as penalties for late papers or missed classes or requirements for a doctor's note, to enforce "self-isolation" and prevent students from venturing out while ill;<ref>{{Cite news|url=https://www.washingtonpost.com/wp-dyn/content/article/2009/08/20/AR2009082003449.html|title=Colleges Warned About Fall Flu Outbreaks on Campus| vauthors = de Vise D |date=20 August 2009|newspaper={{wipe|The Washington Post}}|access-date=17 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007164136/http://www.washingtonpost.com/wp-dyn/content/article/2009/08/20/AR2009082003449.html|archive-date=7 October 2009}}</ref> schools were advised to set aside a room for people developing flu-like symptoms while they waited to go home and to have ill students or staff and those caring for them use face masks.<ref name="AJC8-14">{{Cite news|url=http://www.ajc.com/health/content/shared-auto/healthnews/flu-/630021.html |title=Get Smart About Swine Flu for Back-to-School |date=14 August 2009 |newspaper=Atlanta Journal-Constitution |access-date=31 August 2009 |archive-url=https://web.archive.org/web/20091007164012/http://www.ajc.com/health/content/shared-auto/healthnews/flu-/630021.html |archive-date= 7 October 2009 |url-status=live }}</ref>

In California, school districts and universities were on alert and worked with health officials to launch education campaigns. Many planned to stockpile medical supplies and discuss worst-case scenarios, including plans to provide lessons and meals for low-income children in case elementary and secondary schools closed.<ref name="swine-flu-goes-to-camp"/> [[Organization:University of California|University of California]] campuses stockpiled supplies, from paper masks and hand sanitizer to food and water.<ref name="swine-flu-goes-to-camp">
{{Cite news| url=http://www.latimes.com/features/health/la-me-camp-flu27-2009jul27,0,5762666.story| title=Swine flu goes to camp. Will it go to school next?| vauthors = Mehta S, Santa Cruz N | date=27 July 2009| newspaper={{wipe|Los Angeles Times}}| access-date=17 September 2009| url-status=live| archive-url=https://web.archive.org/web/20090807164438/http://www.latimes.com/features/health/la-me-camp-flu27-2009jul27,0,5762666.story| archive-date=7 August 2009| df=dmy-all}}</ref> To help prepare for contingencies, University of Maryland School of Medicine professor of [[Medicine:Pediatrics|pediatrics]] James C. King Jr. suggested that every county should create an "influenza action team" to be run by the local health department, parents, and school administrators.<ref>{{Cite news |url=https://www.nytimes.com/2009/08/02/opinion/02king.html |title=The ABC's of H1N1| vauthors = King Jr JC |date=1 August 2009|newspaper={{wipe|The New York Times}}|access-date=17 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007164239/http://www.nytimes.com/2009/08/02/opinion/02king.html|archive-date=7 October 2009}}</ref> By 28 October 2009, about 600 schools in the United States had been temporarily closed, affecting over 126,000 students in 19 states.<ref>{{Cite news|url=http://www.foxnews.com/story/0,2933,570129,00.html |title=H1N1 Closes Hundreds of Schools Across the U.S. |date=28 October 2009 |publisher=Fox News |access-date=28 October 2009 |archive-url=https://web.archive.org/web/20091101132151/http://www.foxnews.com/story/0%2C2933%2C570129%2C00.html |archive-date= 1 November 2009 |url-status=live }}</ref>

====Workplace====

Fearing a worst-case scenario, the U.S. Department of Health and Human Services (HHS), the Centers for Disease Control and Prevention and the Department of Homeland Security (DHS) developed updated guidance<ref name="fluguidance"/> and a video for employers to use as they developed plans to respond to the H1N1 outbreak. The guidance suggested that employers consider and communicate their objectives, such as reducing transmission among staff, protecting people who are at increased risk of influenza-related complications from becoming infected, maintaining business operations, and minimising adverse effects on other entities in their [[Supply chain|supply chain]]s.<ref name="fluguidance">{{cite web|url=http://www.flu.gov/plan/workplaceplanning/guidance.html |archive-url=https://web.archive.org/web/20090830005155/http://www.flu.gov/plan/workplaceplanning/guidance.html |archive-date= 30 August 2009 |title=Business Planning |work=Flu.gov |publisher=U.S. Department of Health & Human Services |access-date=17 September 2009 |url-status=live }}</ref>

The CDC estimated that as much as 40% of the workforce might be unable to work at the peak of the pandemic due to the need for many healthy adults to stay home and care for an ill family member,<ref name="LATimes7-25">
{{Cite news| url=http://www.latimes.com/features/health/la-sci-swine-flu25-2009jul25,0,3387335.story| title=Swine flu could kill hundreds of thousands in U.S. if vaccine fails, CDC says|  vauthors = Maugh II TH | date=25 July 2009| newspaper={{wipe|Los Angeles Times}}| access-date=17 September 2009| url-status=live| archive-url=https://web.archive.org/web/20090807165302/http://www.latimes.com/features/health/la-sci-swine-flu25-2009jul25,0,3387335.story| archive-date=7 August 2009| df=dmy-all}}</ref> and advised that individuals should have steps in place should a workplace close down or a situation arise that requires remote work.<ref name="Fox-7-17-09">{{Cite news |url=http://www.foxnews.com/story/0,2933,533591,00.html |title=Swine Flu: Why You Should Still Be Worried | vauthors = Fiore M |date=17 July 2009 |publisher=Fox News |access-date=31 August 2009 |archive-url= https://web.archive.org/web/20090909132519/http://www.foxnews.com/story/0,2933,533591,00.html |archive-date=9 September 2009 |url-status=live }}</ref> The CDC further advised that persons in the workplace should stay home sick for seven days after getting the flu, or 24 hours after symptoms end, whichever is longer.<ref name=24hours/>

In the UK, the [[Organization:Health and Safety Executive|Health and Safety Executive]] (HSE) also issued general guidance for employers.<ref>{{cite web|url=http://www.hse.gov.uk/news/2009/swineflu.htm |title=Swine flu – HSE News Announcement |publisher=Health and Safety Executive |date=18 June 2009 |access-date=31 August 2009 |archive-url=https://web.archive.org/web/20090827061646/http://www.hse.gov.uk/news/2009/swineflu.htm |archive-date=27 August 2009 }}</ref>

====Face masks====

[[File:Swine Flu Masked Train Passengers in Mexico City.jpg|thumb|Mexico City Metro]]
[[File:Influenza-Shock2009(1).jpg|thumb|right|Osaka Loop Line, Japan]]

The U.S. CDC did not recommend the use of face masks or [[Engineering:Respirator|respirator]]s in non-health care settings, such as schools, workplaces, or public places, with a few exceptions: people who were ill with the virus when around other people, and people who were at risk for severe illness while caring for someone with the flu.<ref name="cdcmasks">
{{Cite news|url=http://www.chicagotribune.com/entertainment/chi-tc-health-flu-masks-0923sep27,0,7442578.story |title=Masks may help prevent flu, but aren't advised | vauthors = Roan S |date=27 September 2009 |newspaper=Chicago Tribune |access-date=28 September 2009 |archive-url=https://web.archive.org/web/20091001105331/http://www.chicagotribune.com/entertainment/chi-tc-health-flu-masks-0923sep27%2C0%2C7442578.story |archive-date= 1 October 2009 }}</ref> There was some disagreement about the value of wearing face masks, as some experts feared that masks may have given people a false sense of security and should not have replaced other standard precautions.<ref name="LATimes4-30-09">
{{Cite news|url=http://www.latimes.com/features/health/la-sci-swine-mask30-2009apr30,0,7635596.story |title=Face masks aren't a sure bet against swine flu | vauthors = Roan S, Rong-Gong II L |date=30 April 2009 |newspaper={{wipe|Los Angeles Times}} |access-date=17 September 2009 |archive-url=https://web.archive.org/web/20090503142056/http://www.latimes.com/features/health/la-sci-swine-mask30-2009apr30%2C0%2C7635596.story |archive-date= 3 May 2009 }}</ref>
Yukihiro Nishiyama, professor of virology at Nagoya University's School of Medicine, commented that the masks are "better than nothing, but it's hard to completely block out an airborne virus since it can easily slip through the gaps".<ref name="42bp"/><ref name="fashionnetwork"/>
According to mask manufacturer [[Company:3M|3M]], masks will filter out particles in industrial settings, but "there are no established exposure limits for biological agents such as swine flu virus".<ref name="LATimes4-30-09"/> However, despite the lack of evidence of effectiveness, the use of such masks is common in Asia.<ref name="42bp">{{Cite news|url=https://www.smh.com.au/world/face-masks-part-of-japan-fashion-chic-for-decades-20090504-as48.html|title=Face masks part of Japan fashion chic for decades|date=4 May 2009|work=The Sydney Morning Herald|access-date=7 July 2020|archive-date=8 July 2020|archive-url=https://web.archive.org/web/20200708085652/https://www.smh.com.au/world/face-masks-part-of-japan-fashion-chic-for-decades-20090504-as48.html|url-status=live}}</ref>
<ref name="fashionnetwork">{{Cite news|url=https://ww.fashionnetwork.com/news/Face-masks-part-of-japan-fashion-chic-for-decades,61629.html|archive-url=https://web.archive.org/web/20200703170614/https://ww.fashionnetwork.com/news/Face-masks-part-of-japan-fashion-chic-for-decades,61629.html|archive-date=3 July 2020|title=Face masks part of Japan fashion chic for decades|date=4 May 2009|work=Fashion Network|publisher=FashionNetwork.com|access-date=3 July 2020|agency=AFP}}</ref>
<ref>{{Cite news|url=https://abcnews.go.com/Business/wireStory?id=8870908 |archive-url=https://web.archive.org/web/20091024125412/https://abcnews.go.com/Business/wireStory?id=8870908 |archive-date=24 October 2009 |title=Flu-Related Products May Lift U.S. Makers' Profits | vauthors = Wohl J |date=20 October 2009 |work=ABC News.com |publisher=ABC News |access-date=28 October 2009 |url-status=live }}</ref> They are particularly popular in Japan, where cleanliness and hygiene are highly valued and where etiquette obligates those who are sick to wear masks to avoid spreading disease.<ref name="42bp"/><ref name="fashionnetwork"/>

====Quarantine====

During the height of the fear of a pandemic, some countries initiated or threatened to initiate [[Medicine:Quarantine|quarantine]]s of foreign visitors suspected of having or being in contact with others who may have been infected. In May 2009, the Chinese government confined 21 U.S. students and three teachers to their hotel rooms.<ref>{{Cite news|url=http://www.cnn.com/2009/HEALTH/05/28/us.china.swine.flu/|title=China quarantines U.S. school group over flu concerns|publisher=CNN|date=28 May 2009|access-date=26 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007163940/http://www.cnn.com/2009/HEALTH/05/28/us.china.swine.flu/|archive-date=7 October 2009}}</ref> As a result, the US State Department issued a travel alert about China's anti-flu measures and warned travellers against travelling to China if ill.<ref>{{Cite news|url=http://washingtontimes.com/news/2009/jun/29/kralev-us-warns-travelers-of-chinas-flu-rules/|title=U.S. warns travelers of China's flu rules| vauthors = Kralev N |date=29 June 2009|work=The Washington Times|access-date=26 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007163928/http://washingtontimes.com/news/2009/jun/29/kralev-us-warns-travelers-of-chinas-flu-rules/|archive-date=7 October 2009}}</ref> In Hong Kong, an entire hotel was quarantined with 240 guests;<ref>{{Cite news|url=http://www.taipeitimes.com/News/world/archives/2009/05/03/2003442656|title=Tensions escalate in Hong Kong's swine-flu hotel|date=3 May 2009|work=Taipei Times|access-date=26 September 2009|author=DPA|url-status=live|archive-url=https://web.archive.org/web/20091107164145/http://www.taipeitimes.com/News/world/archives/2009/05/03/2003442656|archive-date=7 November 2009}}</ref> Australia ordered a cruise ship with 2,000 passengers to stay at sea because of a swine flu threat.<ref>{{Cite news|url=http://www.abc.net.au/news/stories/2009/05/28/2583515.htm?section=justin|title=Ship passengers cruisy in swine flu quarantine|date=28 May 2009|work=ABC News|publisher=Australian Broadcasting Corporation|access-date=17 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091007164303/http://www.abc.net.au/news/stories/2009/05/28/2583515.htm?section=justin|archive-date=7 October 2009}}</ref> Egyptian Muslims who went on the annual pilgrimage to [[Place:Mecca|Mecca]] risked being quarantined upon their return.<ref>{{Cite news|url=http://www.philstar.com/breaking-news/469538/egypt-warns-post-hajj-swine-flu-quarantine|title=Egypt warns of post-hajj swine flu quarantine|author=Anonymous|date=21 May 2009|publisher=Philippine Star|access-date=16 October 2013|archive-url=https://web.archive.org/web/20140829044024/http://www.philstar.com/breaking-news/469538/egypt-warns-post-hajj-swine-flu-quarantine|archive-date=29 August 2014}}</ref> Russia and Taiwan said they would quarantine visitors with fevers who come from areas where the flu was present.<ref>{{Cite news|url=http://www.accessmylibrary.com/article-1G1-198655440/swine-flu-cases-europe.html|title=Swine flu cases in Europe; worldwide travel shaken|vauthors=Woolls D|date=27 April 2009|work=The America's Intelligence Wire|access-date=30 January 2011|agency=AP|archive-date=22 August 2019|archive-url=https://web.archive.org/web/20190822021339/https://www.cengage.com/|url-status=live}}</ref> Japan quarantined 47 airline passengers in a hotel for a week in mid-May,<ref>{{Cite news|url=https://www.youtube.com/watch?v=P6fEMGHoQPE|title=Japan Swine Flu Quarantine Ends for Air Passengers|date=17 May 2009|publisher=New Tang Dynasty Television|access-date=26 September 2009|url-status=live|archive-url=https://web.archive.org/web/20091207164750/http://www.youtube.com/watch?v=P6fEMGHoQPE|archive-date=7 December 2009}}</ref> then in mid-June India suggested pre-screening "outbound" passengers from countries thought to have a high rate of infection.<ref>{{Cite news|url=http://www.internationalreporter.com/News-4898/india-wants-us-to-screen-passengers-for-swine-flu.html |title=India wants US to screen passengers for swine flu |date=16 June 2009 |work=International Reporter |access-date=28 September 2009 |author=MIL/Sify.com |archive-url=https://web.archive.org/web/20100923000245/http://internationalreporter.com/News-4898/india-wants-us-to-screen-passengers-for-swine-flu.html |archive-date=23 September 2010 }}</ref>

====Pigs and food safety====


The pandemic virus is a type of swine influenza, derived originally from a strain which lived in pigs, and this origin gave rise to the common name of "swine flu". This term is widely used by mass media, though the Paris-based [[Organization:World Organisation for Animal Health|World Organisation for Animal Health]] as well as industry groups such as the U.S. National Pork Board, the American Meat Institute, and the Canadian Pork Council objected to widespread media use of the name "swine flu" and suggested it should be called "North American flu" instead, while the World Health Organization switched its designation from "swine influenza" to "influenza A (H1N1)" in late April 2009.<ref>{{cite news|url=https://www.france24.com/en/20090501-finally-flu-name-game-resolved-|vauthors=Leela J|title=Finally, the flu name game is resolved|work=France 24|date=1 May 2009|access-date=21 March 2020|archive-date=21 March 2020|archive-url=https://web.archive.org/web/20200321080405/https://www.france24.com/en/20090501-finally-flu-name-game-resolved-|url-status=live}}</ref><ref>{{cite news|url=https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10569528|title=Label 'swine flu' gets up the nose of world's pig farmers|work=The New Zealand Herald|date=30 April 2009|access-date=21 March 2020|archive-date=15 April 2020|archive-url=https://web.archive.org/web/20200415055256/https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10569528|url-status=live}}</ref> The virus has been found in U.S. hogs,<ref>{{Cite news | vauthors = Jordan D | title = Minnesota Pig Tests Positive for H1N1 | url = http://www.newson6.com/Global/story.asp?S=11341148 | publisher = KOTV | date = 20 October 2009 | access-date = 10 December 2009 | url-status = live | archive-url = https://web.archive.org/web/20091207164317/http://www.newson6.com/Global/story.asp?S=11341148 | archive-date = 7 December 2009 | df = dmy-all }}</ref> and Canadian<ref>{{Cite news| vauthors = Evans B | title = News Conference with Minister of Health and Chief Public Health Officer | url = http://www.phac-aspc.gc.ca/alert-alerte/h1n1/conferences/nc-2009-05-02-eng.php|date=5 May 2009|access-date=10 December 2009|publisher=Public Health Agency of Canada| archive-url= https://web.archive.org/web/20091214162720/http://www.phac-aspc.gc.ca/alert-alerte/h1n1/conferences/nc-2009-05-02-eng.php| archive-date= 14 December 2009 | url-status= live}}</ref> as well as in hogs in Northern Ireland, Argentina, and Norway.<ref>{{cite web|title=Human swine flu in pigs|url=http://scienceblogs.com/effectmeasure/2009/10/human_swine_flu_in_pigs.php|date=20 October 2009|access-date=10 December 2009|work=Effect Measure|archive-url=https://web.archive.org/web/20091108035341/http://scienceblogs.com/effectmeasure/2009/10/human_swine_flu_in_pigs.php|archive-date=8 November 2009}}</ref> Leading health agencies and the United States Secretary of Agriculture have stressed that eating properly cooked pork or other food products derived from pigs will not cause flu.<ref name=CDC_pork>{{cite press release|url=https://www.who.int/mediacentre/news/statements/2009/h1n1_20090430/en/index.html |title=Joint FAO/WHO/OIE Statement on influenza A(H1N1) and the safety of pork|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |date=7 May 2009|access-date=26 September 2009| archive-url= https://web.archive.org/web/20090923210159/http://www.who.int/mediacentre/news/statements/2009/h1n1_20090430/en/index.html| archive-date= 23 September 2009 }}</ref><ref>{{Cite news | vauthors = Desmon S |title=Pork industry rues swine flu |work=The Baltimore Sun |volume=172 |issue=233 |date=21 August 2009 |pages=1, 16 |url=http://www.baltimoresun.com/health/bal-md.hs.flu21aug21,0,5219798.story |access-date=1 September 2009 |archive-url= https://web.archive.org/web/20091007164004/http://www.baltimoresun.com/health/bal-md.hs.flu21aug21,0,5219798.story |archive-date=7 October 2009 }}</ref> Nevertheless, on 27 April [[Place:Azerbaijan|Azerbaijan]] imposed a ban on the importation of animal husbandry products from the entire [[Earth:Americas|Americas]].<ref name="trend">{{Cite news |url=http://news-en.trend.az/society/health/1462401.html |archive-url=https://web.archive.org/web/20090716080338/http://news-en.trend.az/society/health/1462401.html |archive-date=16 July 2009 |publisher=Trend News Agency |title=Prevention against "swine flu" stabile in Azerbaijan: minister |date=28 April 2009 |access-date=28 April 2009 }}</ref> The Indonesian government also halted the importation of pigs and initiated the examination of {{Nowrap|9 million}} pigs in Indonesia.<ref>{{cite web|url=http://nasional.kompas.com/read/xml/2009/04/27/11120229/cegah.flu.babi.pemerintah.gelar.rapat.koordinasi|title=Cegah flu babi, pemerintah gelar rapat koordinasi|date=27 April 2009|work=Kompas newspaper|access-date=14 April 2011 |url-status=live|archive-url=https://web.archive.org/web/20090501235943/http://nasional.kompas.com/read/xml/2009/04/27/11120229/cegah.flu.babi.pemerintah.gelar.rapat.koordinasi|archive-date=1 May 2009}}</ref> The Egyptian government ordered the slaughter of all pigs in Egypt on 29 April.<ref>{{Cite news|url=http://www.abc.net.au/news/stories/2009/04/30/2556545.htm?section=justin%20Egypt%20orders%20pig%20cull|agency=AFP|title=Egypt orders pig cull|work=ABC News|publisher=Australian Broadcasting Corporation|access-date=27 September 2009|date=30 April 2009|url-status=live|archive-url=https://web.archive.org/web/20091007163932/http://www.abc.net.au/news/stories/2009/04/30/2556545.htm?section=justin%20Egypt%20orders%20pig%20cull|archive-date=7 October 2009}}</ref>

==Treatment==

{{Further|Biology:Influenza treatment}}

A number of methods have been recommended to help ease symptoms, including adequate liquid intake and rest.<ref name=Mayo>{{cite web |url=http://www.mayoclinic.com/health/influenza/DS00081/DSECTION=treatments-and-drugs |author=Mayo Clinic Staff |publisher=Mayo Clinic |work=Diseases and Conditions |title=Influenza (flu) treatments and drugs |access-date=20 May 2009 |archive-url=https://web.archive.org/web/20091007164218/http://www.mayoclinic.com/health/influenza/DS00081/DSECTION=treatments-and-drugs |archive-date=7 October 2009 |url-status=live }}</ref> [[Medicine:Over-the-counter drug|Over-the-counter]] pain medications such as [[Chemistry:Paracetamol|paracetamol]] and [[Chemistry:Ibuprofen|ibuprofen]] do not kill the virus; however, they may be useful to reduce symptoms.<ref name="Medline"/> [[Chemistry:Aspirin|Aspirin]] and other salicylate products should not be used by people under 16 with any flu-type symptoms because of the risk of developing Reye's Syndrome.<ref name="National Reyes Syndrome Foundation">{{cite web |url=http://www.reyessyndrome.org/aspirin.html |publisher=National Reye's Syndrome Foundation |title=Aspirin / Salicylates and Reyes Syndrome |access-date=14 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110719104914/http://www.reyessyndrome.org/aspirin.html |archive-date=19 July 2011 |year=1974 }}</ref>

If the fever is mild and there are no other complications, fever medication is not recommended.<ref name="Medline">{{cite encyclopedia|url=https://www.nlm.nih.gov/medlineplus/ency/article/003090.htm |publisher=U.S. National Library of Medicine |encyclopedia=Medline Plus Medical Encyclopedia |title=Fever |access-date=20 May 2009| archive-url= https://web.archive.org/web/20090515173838/http://www.nlm.nih.gov/medlineplus/ency/article/003090.htm| archive-date= 15 May 2009 | url-status= live}}</ref> Most people recover without medical attention, although ones with pre-existing or underlying medical conditions are more prone to complications and may benefit from further treatments.<ref name=CDC_symptoms>{{cite web |url=https://www.cdc.gov/h1n1flu/sick.htm |publisher=Centers for Disease Control and Prevention (CDC) |title=What To Do If You Get Sick: 2009 H1N1 and Seasonal Flu |date=7 May 2009 |access-date=1 November 2009 |archive-url=https://web.archive.org/web/20091103195142/http://www.cdc.gov/h1n1flu/sick.htm |archive-date=3 November 2009 }}</ref>

People in at-risk groups should be treated with antivirals (oseltamivir or zanamivir) as soon as possible when they first experience flu symptoms. The at-risk groups include pregnant and post partum women, children under two years old, and people with underlying conditions such as respiratory problems.<ref name=CDCOct16AntiviralRecommendations/> People who are not in an at-risk group who have persistent or rapidly worsening symptoms should also be treated with antivirals. People who have developed [[Medicine:Pneumonia#Diagnosis|pneumonia]] should be given both antivirals and antibiotics, as in many severe cases of H1N1-caused illness, bacterial infection develops.<ref name="NikkiShindoNov12">{{Cite news|url=https://www.who.int/mediacentre/vpc_transcript_12_november_09_nikki_shindo.pdf|title=Transcript of virtual press conference with Gregory Hartl, Spokesperson for H1N1, and Dr Nikki Shindo, Medical Officer, Global Influenza Programme, World Health Organization|date=12 November 2009|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=18 November 2009| archive-url= https://web.archive.org/web/20091129180559/http://www.who.int/mediacentre/vpc_transcript_12_november_09_nikki_shindo.pdf| archive-date= 29 November 2009 | url-status= live}}</ref> Antivirals are most useful if given within 48 hours of the start of symptoms and may improve outcomes in hospitalised patients.<ref>{{cite journal | vauthors = Jain S, Kamimoto L, Bramley AM, Schmitz AM, Benoit SR, Louie J, Sugerman DE, Druckenmiller JK, Ritger KA, Chugh R, Jasuja S, Deutscher M, Chen S, Walker JD, Duchin JS, Lett S, Soliva S, Wells EV, Swerdlow D, Uyeki TM, Fiore AE, Olsen SJ, Fry AM, Bridges CB, Finelli L | display-authors = 6 | title = Hospitalized patients with 2009 H1N1 influenza in the United States, April–June 2009 | journal = The New England Journal of Medicine | volume = 361 | issue = 20 | pages = 1935–44 | date = November 2009 | pmid = 19815859 | doi = 10.1056/NEJMoa0906695 | citeseerx = 10.1.1.183.7888 }}</ref> In those beyond 48 hours who are moderately or severely ill, antivirals may still be beneficial.<ref name="cdc.gov"/> If [[Chemistry:Oseltamivir|oseltamivir]] (Tamiflu) is unavailable or cannot be used, [[Chemistry:Zanamivir|zanamivir]] (Relenza) is recommended as a substitute.<ref name=CDCOct16AntiviralRecommendations>{{cite web|url=https://www.cdc.gov/h1n1flu/recommendations.htm|title=Updated Interim Recommendations for the Use of Antiviral Medications in the Treatment and Prevention of Influenza for the 2009–2010 Season|date=8 September 2009|publisher=Centers for Disease Control and Prevention (CDC) |access-date=17 September 2009| archive-url= https://web.archive.org/web/20090917030300/http://www.cdc.gov/h1n1flu/recommendations.htm| archive-date= 17 September 2009 | url-status= live}}</ref><ref name="WHOAug21antiviralrecommendations">{{cite web|url=https://www.who.int/csr/disease/swineflu/notes/h1n1_use_antivirals_20090820/en/index.html|title=Recommended use of antivirals|date=21 August 2009|work=Global Alert and Response (GAR)|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=6 November 2009|location=Geneva| archive-url= https://web.archive.org/web/20091104171524/http://www.who.int/csr/disease/swineflu/notes/h1n1_use_antivirals_20090820/en/index.html| archive-date= 4 November 2009 }}</ref> [[Chemistry:Peramivir|Peramivir]] is an experimental [[Biology:Antiviral drug|antiviral drug]] approved for hospitalised patients in cases where the other available methods of treatment are ineffective or unavailable.<ref>{{Cite news|url=http://www.prnewswire.com/news-releases/emergency-use-authorization-granted-for-biocrysts-peramivir-65876107.html|title=Emergency Use Authorization Granted For BioCryst's Peramivir|date=23 October 2009|publisher=PRNewswire|access-date=30 January 2011|agency=PRNewswire-FirstCall|archive-url=https://web.archive.org/web/20110606053938/http://www.prnewswire.com/news-releases/emergency-use-authorization-granted-for-biocrysts-peramivir-65876107.html|archive-date=6 June 2011}}</ref>

To help avoid shortages of these drugs, the U.S. CDC recommended oseltamivir treatment primarily for people hospitalised with pandemic flu; people at risk of serious flu complications due to underlying medical conditions; and patients at risk of serious flu complications. The CDC warned that the indiscriminate use of antiviral medications to prevent and treat influenza could ease the way for drug-resistant strains to emerge, which would make the fight against the pandemic that much harder. In addition, a British report found that people often failed to complete a full course of the drug or took the medication when not needed.<ref name=Tami1>{{Cite news |url=http://www.washingtonexaminer.com/economy/ap/53932902.html |archive-url=https://web.archive.org/web/20091007164124/http://www.washingtonexaminer.com/economy/ap/53932902.html |archive-date=7 October 2009 |title=WHO: Healthy people who get swine flu don't need Tamiflu; drug for young, old, pregnant | vauthors = Cheng M |date=21 August 2009 |work=Washington Examiner |access-date=17 September 2009 }}</ref>

===Side effects===

Both medications mentioned above for treatment, oseltamivir and zanamivir, have known side effects, including lightheadedness, chills, nausea, vomiting, loss of appetite, and trouble breathing. Children were reported to be at increased risk of self-injury and confusion after taking oseltamivir.<ref name=Mayo/> The WHO warned against buying antiviral medications from online sources and estimated that half the drugs sold by online pharmacies without a physical address were counterfeit.<ref name=onlinedrugs>{{Cite news |url=http://www.webmd.boots.com/cold-and-flu/news/20090508/warning-against-buying-flu-drugs-online |title=Warning against buying flu drugs online |publisher=Boots |author=BMJ Group |date=8 May 2009 |access-date=16 October 2013 |archive-url=https://web.archive.org/web/20131017005011/http://www.webmd.boots.com/cold-and-flu/news/20090508/warning-against-buying-flu-drugs-online |archive-date=17 October 2013 }}</ref>

===Resistance===

In December 2012, the [[Organization:World Health Organization|World Health Organization]] (WHO) reported 314 samples of the 2009 pandemic H1N1 flu tested worldwide have shown resistance to [[Chemistry:Oseltamivir|oseltamivir]] (Tamiflu).<ref name="Update on oseltamivir resistance to influenza H1N1 (2009) viruses">
{{cite web
 |url=https://www.who.int/csr/disease/influenza/2010_12_15_weekly_web_update_oseltamivir_resistance.pdf
 |archive-url=https://web.archive.org/web/20110127001912/http://www.who.int/csr/disease/influenza/2010_12_15_weekly_web_update_oseltamivir_resistance.pdf
 |archive-date=27 January 2011
|title=Update on oseltamivir resistance to influenza H1N1 (2009) viruses
 |publisher=[[Organization:World Health Organization|World Health Organization]] (WHO)
 |date=15 December 2010
|access-date=30 December 2010
|url-status=live
}}</ref> It is not totally unexpected as 99.6% of the seasonal H1N1 flu strains tested have developed resistance to oseltamivir.<ref>
{{cite web| url=https://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly39.htm| title=2008–2009 Influenza Season Week 39 ending October 3, 2009| publisher=Centers for Disease Control and Prevention (CDC)| date=9 October 2009| access-date=20 November 2009| archive-url= https://web.archive.org/web/20100306015519/http://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly39.htm| archive-date=6 March 2010| url-status= live}}</ref> No circulating flu has yet shown any resistance to [[Chemistry:Zanamivir|zanamivir]] (Relenza), the other available anti-viral.<ref name=autogenerated1>{{cite web| url=https://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly32.htm| title=2008–2009 Influenza Season Week 32 ending August 15, 2009| work=Flu Activity & Surveillance| publisher=Centers for Disease Control and Prevention (CDC) | date=21 August 2009 |access-date=4 December 2009| archive-url= https://web.archive.org/web/20091103235431/http://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly32.htm| archive-date= 3 November 2009 | url-status= live}}</ref>

===Antivirals effectiveness questioned in healthy adults===

On 8 December 2009, the Cochrane Collaboration, which reviews medical evidence, announced in a review published in ''BMJ'' that it had reversed its previous findings that the antiviral drugs [[Chemistry:Oseltamivir|oseltamivir]] (Tamiflu) and [[Chemistry:Zanamivir|zanamivir]] (Relenza) can ward off [[Medicine:Pneumonia|pneumonia]] and other serious conditions linked to influenza. They reported that an analysis of 20 studies showed oseltamivir offered mild benefits for healthy adults if taken within 24 hours of onset of symptoms, but found no clear evidence it prevented lower respiratory tract infections or other [[Medicine:Complication|complications]] of influenza.<ref>{{Cite news|url=https://www.bloomberg.com/apps/news?pid=20601100&sid=a9acc7qIJFdU|archive-url=https://web.archive.org/web/20100107164229/http://www.bloomberg.com/apps/news?pid=20601100&sid=a9acc7qIJFdU|archive-date=7 January 2010|title=Roche's Tamiflu Not Proven to Cut Flu Complications, Study Says| vauthors = Cortez MF |date=9 December 2009|publisher=Bloomberg L.P|access-date=11 December 2009}}</ref><ref name=BMJDec8>{{cite journal | vauthors = Jefferson T, Jones M, Doshi P, Del Mar C | title = Neuraminidase inhibitors for preventing and treating influenza in healthy adults: systematic review and meta-analysis | journal = BMJ | volume = 339 | pages = b5106 | date = December 2009 | pmid = 19995812 | pmc = 2790574 | doi = 10.1136/bmj.b5106 }}</ref> Of note, their published finding related only to use in healthy adults with influenza but not in patients judged to be at high risk of complications (pregnant women, children under five and those with underlying medical conditions), and uncertainty over its role in reducing complications in healthy adults still left it as a useful drug for reducing the duration of symptoms. In general, the Cochrane Collaboration concluded "Paucity of good data".<ref name=BMJDec8/><ref>{{Cite journal| vauthors = Godlee F |date=10 December 2009|title=We want raw data, now |journal=BMJ |url=http://www.bmj.com/cgi/content/full/339/dec10_2/b5405|doi=10.1136/bmj.b5405 |volume=339 |pages=b5405 |s2cid=72788639 |access-date=14 April 2011 |url-status=live|archive-url=https://web.archive.org/web/20100307163921/http://www.bmj.com/cgi/content/full/339/dec10_2/b5405 |archive-date=7 March 2010}}</ref>

==Epidemiology==
{| class="wikitable" style="max-width: 22em; float: right; clear: right; margin:0 0 0.5em 1em;" 
|+ 2009 flu pandemic data<ref name="WHO100"/>
|-
! Area
! Lab confirmed<br />deaths reported to the WHO
|-
|style="text-align:left"| '''Worldwide (total)'''
|'''At least 18,449'''
|-
|style="text-align:left"| Africa
|168
|- 
|style="text-align:left"| Americas
|At least 8,533
|- 
|style="text-align:left"| Eastern Mediterranean
|1,019
|- 
|style="text-align:left"| Europe
|At least 4,079
|- 
|style="text-align:left"| South-East Asia
|1,992
|- 
|style="text-align:left"| Western Pacific
|1,858
|-
|colspan="2"|Further information: Cases and deaths by country

<small>Note: The ratio of confirmed deaths to total deaths due to the pandemic is unknown. For more information, see "Data reporting and accuracy".</small>
|}
While it is not known precisely where or when the virus originated,<ref name=Nature>{{cite journal | vauthors = Butler D | title = How severe will the flu outbreak be? | journal = Nature | volume = 459 | issue = 7243 | pages = 14–15 | date = May 2009 | pmid = 19424121 | pmc = 7095218 | doi = 10.1038/459014a }}</ref><ref>{{cite video |people= Larry Pope |title= Smithfield Foods CEO on flu virus |url= http://www.nbcnews.com/id/21134540 |format= flv |medium= Television production |publisher= NBC News |access-date= 16 September 2009 |df= dmy-all |archive-date= 29 January 2013 |archive-url= https://web.archive.org/web/20130129090138/http://www.nbcnews.com/id/21134540/ |url-status= live }}</ref> analyses in scientific journals have suggested that the H1N1 strain responsible for the 2009 outbreak first evolved in September 2008 and circulated amongst humans for several months, before being formally recognised and identified as a novel strain of influenza.<ref name=Nature/><ref name="circles globe">{{cite journal | vauthors = Cohen J, Enserink M | title = Infectious diseases. As swine flu circles globe, scientists grapple with basic questions | journal = Science | volume = 324 | issue = 5927 | pages = 572–73 | date = May 2009 | pmid = 19407164 | doi = 10.1126/science.324_572 }}</ref>

===Mexico===


The virus was first reported in two U.S. children in March 2009, but health officials have reported that it apparently infected people as early as January 2009 in Mexico.<ref name="Reuters6-11">{{Cite news |url=https://www.reuters.com/article/middleeastCrisis/idUSN11399103 |title=New flu has been around for years in pigs – study | vauthors = Fox M |date=11 June 2009 |work=Reuters |access-date=17 September 2009 |url-status=live |archive-url=https://web.archive.org/web/20091007164740/http://www.reuters.com/article/middleeastCrisis/idUSN11399103 |archive-date=7 October 2009 }}</ref> The outbreak was first identified in [[Place:Mexico City|Mexico City]] on 18 March 2009;<ref name="NYT4-26">{{Cite news|url=https://www.nytimes.com/2009/04/27/health/27questions.html|title=Flu Outbreak Raises a Set of Questions| vauthors = McNeil Jr DG |date=26 April 2009|work=The New York Times|access-date=17 September 2009|url-status=live|archive-url=https://web.archive.org/web/20110215221659/http://www.nytimes.com/2009/04/27/health/27questions.html|archive-date=15 February 2011}}</ref> immediately after the outbreak was officially announced, Mexico notified the U.S. and World Health Organization, and within days of the outbreak Mexico City was "effectively shut down".<ref name="Fox5-12">{{Cite news|url=http://www.foxnews.com/story/0,2933,519902,00.html|title=Study: Mexico Has Thousands More Swine Flu Cases|date=12 May 2009|publisher=Fox News Network|access-date=17 September 2009|agency=Associated Press|url-status=live|archive-url=https://web.archive.org/web/20090924031546/http://www.foxnews.com/story/0,2933,519902,00.html|archive-date=24 September 2009}}</ref> Some countries cancelled flights to Mexico while others halted trade. Calls to close the border to contain the spread were rejected.<ref name="Fox5-12"/> Mexico already had hundreds of non-lethal cases before the outbreak was officially discovered, and was therefore in the midst of a "silent epidemic". As a result, Mexico was reporting only the most serious cases which showed more severe signs different from those of normal flu, possibly leading to a skewed initial estimate of the case fatality rate.<ref name="NYT4-26"/>

===United States===

[[File:Barack Obama Meeting Swine Flu Outbreak 5-1.jpg|thumb|President Obama at Homeland Security Council meeting in Cabinet Room to discuss the H1N1 flu on 1 May 2009]]

The new strain was first identified by the CDC in two children, neither of whom had been in contact with pigs. The first case, from San Diego County, California, was confirmed from clinical specimens (nasopharyngeal [[Engineering:Cotton swab|swab]]) examined by the CDC on 14 April 2009. A second case, from nearby Imperial County, California, was confirmed on 17 April. The patient in the first confirmed case had flu symptoms including fever and cough upon clinical examination on 30 March and the second on 28 March.<ref name="MMWR" />

The first confirmed H1N1/09 pandemic flu death, which occurred at Texas Children's Hospital in Houston, Texas, was of a toddler from Mexico City who was visiting family in Brownsville, Texas, before being air-lifted to Houston for treatment.<ref>{{Cite news|url=http://abclocal.go.com/ktrk/story?section=news/health&id=6786416|title=Swine flu victim dies in Houston|date=29 April 2009|publisher=KTRK-TV/DT|access-date=18 November 2009|location=Houston, Texas|agency=AP|archive-url=https://web.archive.org/web/20091107164755/http://abclocal.go.com/ktrk/story?section=news%2Fhealth&id=6786416&rss=rss-ktrk-article-6786416|archive-date=7 November 2009}}</ref> The Infectious Diseases Society of America estimated that the total number of deaths in the U.S. was 12,469.<ref>{{cite journal | vauthors = Shrestha SS, Swerdlow DL, Borse RH, Prabhu VS, Finelli L, Atkins CY, Owusu-Edusei K, Bell B, Mead PS, Biggerstaff M, Brammer L, Davidson H, Jernigan D, Jhung MA, Kamimoto LA, Merlin TL, Nowell M, Redd SC, Reed C, Schuchat A, Meltzer MI | display-authors = 6 | title = Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States (April 2009 – April 2010) | journal = Clinical Infectious Diseases | volume = 52 | issue = Suppl 1 | pages = S75–82 | date = January 2011 | pmid = 21342903 | doi = 10.1093/cid/ciq012 | doi-access = free }}</ref>

===Data reporting and accuracy<!-- This section is linked to by Template:2009 flu pandemic menu -->===


Influenza surveillance information "answers the questions of where, when, and what influenza viruses are circulating. Sharing of such information is especially crucial during an emergent pandemic as in April 2009, when the genetic sequences of the initial viruses were rapidly and openly shared via the GISAID Initiative within days of identification,<ref>{{Cite news|url=http://www.cidrap.umn.edu/news-perspective/2009/06/pandemic-reveals-strengths-new-flu-database|title=Pandemic reveals strengths of new flu database|work=CIDRAP|access-date=2017-09-06|archive-date=6 September 2017|archive-url=https://web.archive.org/web/20170906092102/http://www.cidrap.umn.edu/news-perspective/2009/06/pandemic-reveals-strengths-new-flu-database|url-status=live}}</ref> playing a key role in facilitating an early response to the evolving pandemic.<ref>{{cite journal | vauthors = Elbe S, Buckland-Merrett G | title = Data, disease and diplomacy: GISAID's innovative contribution to global health | journal = Global Challenges | volume = 1 | issue = 1 | pages = 33–46 | date = January 2017 | pmid = 31565258 | pmc = 6607375 | doi = 10.1002/gch2.1018 | bibcode = 2017GloCh...1...33E }}</ref><ref>{{Cite news|url=https://www.scientificamerican.com/article/better-flu-vaccine/|title=Building a Better Flu Vaccine – and Giving Chickens a Rest| vauthors = Greenemeier L |work=Scientific American|access-date=2017-09-06|url-status=live|archive-url=https://web.archive.org/web/20170906093448/https://www.scientificamerican.com/article/better-flu-vaccine/|archive-date=6 September 2017|df=dmy-all}}</ref><ref>{{cite journal | vauthors = McCauley JW | title = Viruses: Model to accelerate epidemic responses | journal = Nature | volume = 542 | issue = 7642 | pages = 414 | date = February 2017 | pmid = 28230113 | doi = 10.1038/542414b | bibcode = 2017Natur.542..414M | doi-access = free }}</ref> Surveillance is used to determine if influenza activity is increasing or decreasing, but cannot be used to ascertain how many people have become ill with influenza."<ref>{{cite web|url=https://www.cdc.gov/flu/weekly/fluactivity.htm|title=Flu Activity & Surveillance|date=30 November 2009|publisher=Centers for Disease Control and Prevention (CDC) |access-date=2 December 2009| archive-url= https://web.archive.org/web/20091126143103/http://www.cdc.gov/flu/weekly/fluactivity.htm| archive-date= 26 November 2009 | url-status= live}}</ref> For example, as of late June, influenza surveillance information showed the U.S. had nearly 28,000 laboratory-confirmed cases including 3,065 hospitalizations and 127 deaths. But mathematical modelling showed an estimated {{Nowrap|1 million}} Americans had the 2009 pandemic flu at the time, according to Lyn Finelli, a flu surveillance official with the CDC.<ref>{{Cite news|url=http://www.huffingtonpost.com/2009/06/25/us-swine-flu-cases-may-ha_n_221240.html|title=US Swine Flu Cases May Have Hit 1 Million| vauthors = Stobbe M |date=25 June 2009|work=The Huffington Post|access-date=2 December 2009|agency=AP|url-status=live|archive-url=https://web.archive.org/web/20091207163916/http://www.huffingtonpost.com/2009/06/25/us-swine-flu-cases-may-ha_n_221240.html|archive-date=7 December 2009}}</ref> Estimating deaths from influenza is also a complicated process. In 2005, influenza only appeared on the death certificates of 1,812 people in the US. The average annual US death toll from flu is, however, estimated to be 36,000.<ref name="Ellenberg">{{Cite news|url=http://www.slate.com/id/2218367/ |title=Influenza Body Count | vauthors = Ellenberg J |date=4 July 2009 |work=Slate |access-date=2 December 2009 |archive-url=https://web.archive.org/web/20100115031919/http://www.slate.com/id/2218367/ |archive-date=15 January 2010 |url-status=live }}</ref> The CDC explains:<ref>{{cite web|url=https://www.cdc.gov/flu/about/disease/us_flu-related_deaths.htm|title=Questions and Answers Regarding Estimating Deaths from Influenza in the United States|date=4 September 2009|work=Seasonal Influenza (Flu)|publisher=Centers for Disease Control and Prevention (CDC)|access-date=2 December 2009|url-status=live|archive-url=https://web.archive.org/web/20091201101247/http://www.cdc.gov/flu/about/disease/us_flu-related_deaths.htm|archive-date=1 December 2009}}</ref> "[I]nfluenza is infrequently listed on death certificates of people who die from flu-related complications" and hence, "Only counting deaths where influenza was included on a death certificate would be a gross underestimation of influenza's true impact."

Influenza surveillance information on the 2009 H1N1 flu pandemic is available, but almost no studies attempted to estimate the total number of deaths attributable to H1N1 flu. Two studies were carried out by the CDC; the later of them estimated that between 7,070 and 13,930 deaths were attributable to H1N1 flu from April to 14 November 2009.<ref name="WHOestimateNov">{{cite web|url=https://www.cdc.gov/h1n1flu/estimates_2009_h1n1.htm|title=CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April – December 12, 2009|date=15 January 2010|work=H1N1 Flu|publisher=CDC|access-date=17 January 2010| archive-url= https://web.archive.org/web/20100119201425/http://www.cdc.gov/h1n1flu/estimates_2009_h1n1.htm| archive-date= 19 January 2010 | url-status= live}}</ref> During the same period, 1,642 deaths were officially confirmed as caused by H1N1 flu.<ref>{{cite web|url=https://www.cdc.gov/flu/weekly/weeklyarchives2009-2010/weekly45.htm|title=CDC week 45|date=14 November 2009|publisher=Centers for Disease Control and Prevention (CDC)|access-date=14 December 2009|url-status=live|archive-url=https://web.archive.org/web/20100107165447/http://www.cdc.gov/flu/weekly/weeklyarchives2009-2010/weekly45.htm|archive-date=7 January 2010}}</ref><ref>{{cite web|url=https://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly34.htm|title=CDC week 34|date=30 August 2009|publisher=Centers for Disease Control and Prevention (CDC)|access-date=14 December 2009|url-status=live|archive-url=https://web.archive.org/web/20100107165933/http://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly34.htm|archive-date=7 January 2010}}</ref> The WHO stated in 2010 that total mortality (including unconfirmed or unreported deaths) from H1N1 flu was "unquestionably higher" than their own confirmed death statistics.<ref name="WHOstatement">{{Cite news|url=https://www.bloomberg.com/apps/news?pid=20601124&sid=akODnI6cicxw|archive-url=https://web.archive.org/web/20100107163907/http://www.bloomberg.com/apps/news?pid=20601124&sid=akODnI6cicxw|archive-date=7 January 2010| vauthors = Gale J |title=Clinical features of severe cases of pandemic influenza|date=4 December 2009|publisher=[[Company:Bloomberg L.P.|Bloomberg]]|access-date=4 December 2009}}</ref>

The initial outbreak received a week of near-constant media attention. Epidemiologists cautioned that the number of cases reported in the early days of an outbreak can be very inaccurate and deceptive, due to several causes, among them [[Selection bias|selection bias]], [[Social:Media bias|media bias]] and incorrect reporting by governments. Inaccuracies could also be caused by authorities in different countries looking at differing population groups. Furthermore, countries with poor health care systems and older laboratory facilities may take longer to identify or report cases.<ref>{{Cite news |agency={{wipe|Reuters}} |url=http://www.alertnet.org/thenews/newsdesk/IRIN/2687d9562c28b487c30aca4e889c5e93.htm |title= No A (H1N1) cases – reality or poor lab facilities? |date=8 May 2009 |access-date=9 May 2009| archive-url= https://web.archive.org/web/20090511130411/http://www.alertnet.org/thenews/newsdesk/IRIN/2687d9562c28b487c30aca4e889c5e93.htm| archive-date= 11 May 2009 | url-status= live}}</ref> "[E]ven in developed countries the [numbers of flu deaths] are uncertain, because medical authorities don't usually verify who actually died of influenza and who died of a flu-like illness".<ref>{{Cite journal| vauthors = Sandman P, Lanard J |year=2005|title=Bird Flu: Communicating the Risk|journal=Perspectives in Health Magazine|volume=10|issue=2|url=http://www.paho.org/English/DD/PIN/Number22_article1a.htm|access-date=14 April 2011 |url-status=live|archive-url=https://web.archive.org/web/20110423155359/http://www.paho.org/English/DD/PIN/Number22_article1a.htm|archive-date=23 April 2011}}</ref> Joseph S. Bresee, then CDC flu division's epidemiology chief and Michael Osterholm, director of the [[Organization:Center for Infectious Disease Research and Policy|Center for Infectious Disease Research and Policy]] pointed out that millions of people have had H1N1 flu, usually in a mild form, so the numbers of laboratory-confirmed cases were actually meaningless, and in July 2009, the WHO stopped keeping count of individual cases and focused more on major outbreaks.<ref name="WHO-Briefing-Note-3">{{cite web|url=https://www.who.int/csr/disease/swineflu/notes/h1n1_surveillance_20090710/en/index.html|title=Pandemic (H1N1) 2009 briefing note 3|publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=17 July 2009| archive-url= https://web.archive.org/web/20090722084146/http://www.who.int/csr/disease/swineflu/notes/h1n1_surveillance_20090710/en/index.html| archive-date= 22 July 2009 }}</ref>

===Follow-up===

A Wisconsin study published in the ''Journal of the American Medical Association'' in September 2010, reported that findings showed that the 2009 H1N1 flu was no more severe than the seasonal flu. "The risk of most serious complications was not elevated in adults or children", the study's authors wrote. "Children were disproportionately affected by 2009 H1N1 infection, but the perceived severity of symptoms and risk of serious outcomes were not increased." Children infected in the 2009 H1N1 flu pandemic were no more likely to be hospitalized with complications or get pneumonia than those who catch seasonal strains. About 1.5% of children with the H1N1 swine flu strain were hospitalized within 30 days, compared with 3.7% of those sick with a seasonal strain of H1N1 and 3.1% with an H3N2 virus.<ref>{{cite web | vauthors = Randall T |url=https://www.bloomberg.com/news/2010-09-07/swine-flu-in-children-is-no-more-severe-than-seasonal-virus-study-finds.html |title=Swine Flu Found No More Severe Than Seasonal Virus |publisher=Bloomberg |date=7 September 2010 |access-date=24 May 2011 |url-status=live |archive-url=https://web.archive.org/web/20100910101552/http://www.bloomberg.com/news/2010-09-07/swine-flu-in-children-is-no-more-severe-than-seasonal-virus-study-finds.html |archive-date=10 September 2010 }}</ref>

CDC illness and death estimates from April 2009 to April 2010, in the US are as follows:
* between 43&nbsp;million and 89&nbsp;million cases of 2009 H1N1 occurred between April 2009 and 10 April 2010. The mid-level in this range is about 61&nbsp;million people infected with 2009 H1N1.
* between about 195,000 and 403,000 H1N1-related hospitalizations occurred between April 2009 and 10 April 2010. The mid-level in this range is about 274,000 2009 H1N1-related hospitalizations.
* between about 8,870 and 18,300 2009 H1N1-related deaths occurred between April 2009 and 10 April 2010. The mid-level in this range is about 12,470 2009 H1N1-related deaths.<ref>{{cite web |url=https://www.cdc.gov/h1n1flu/estimates_2009_h1n1.htm |title=Updated CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April 2009 – April 10, 2010 |publisher=Cdc.gov |access-date=24 May 2011 |url-status=live |archive-url=https://web.archive.org/web/20101209191245/http://www.cdc.gov/h1n1flu/estimates_2009_h1n1.htm |archive-date=9 December 2010 }}</ref><ref name="Estimating the Burden of 2009 Pandemic Influenza A (H1N1) in the United States (April 2009 – April 2010)">{{cite journal | vauthors = Shrestha SS, Swerdlow DL, Borse RH, Prabhu VS, Finelli L, Atkins CY, Owusu-Edusei K, Bell B, Mead PS, Biggerstaff M, Brammer L, Davidson H, Jernigan D, Jhung MA, Kamimoto LA, Merlin TL, Nowell M, Redd SC, Reed C, Schuchat A, Meltzer MI | display-authors = 6 | title = Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States (April 2009 – April 2010) | journal = Clinical Infectious Diseases | volume = 52 | issue = Suppl 1 | pages = S75-82 | date = January 2011 | pmid = 21342903 | doi = 10.1093/cid/ciq012 | doi-access = free }}</ref><ref name="Flu Season Continues; Severity Indicators Rise">{{cite web | url=https://www.cdc.gov/flu/news/flu-severity-rises.htm | title=Flu Season Continues; Severity Indicators Rise | publisher=Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD) | date=17 January 2014 | access-date=24 January 2014 | url-status=live | archive-url=https://web.archive.org/web/20140121064156/http://www.cdc.gov/flu/news/flu-severity-rises.htm | archive-date=21 January 2014 | df=dmy-all }}</ref>

It has been stated that about 36,000 die from the seasonal flu in the U.S. each year,<ref>{{cite news | vauthors = Mason M |url= http://health.nytimes.com/health/guides/disease/the-flu/overview.html |title=Flu – Symptoms, Diagnosis, Treatment of Flu – NY Times Health Information |access-date=24 May 2011 |url-status=live |archive-url=https://web.archive.org/web/20110714223418/http://health.nytimes.com/health/guides/disease/the-flu/overview.html |archive-date=14 July 2011 |newspaper=The New York Times }}</ref> and this is frequently understood as an indication that the H1N1 strain was not as severe as seasonal influenza. The 36,000 estimate was presented in a 2003 study by CDC scientists and refers to a period from 1990 to 1991 through 1998–99. During those years, the number of estimated deaths ranged from 17,000 to 52,000, with an average of about 36,000. Throughout that decade, influenza A (H3N2) was the predominant virus during most of the seasons, and H3N2 influenza viruses are typically associated with higher death rates. The ''JAMA'' study also looked at seasonal influenza-associated deaths over a 23-year period, from 1976 to 1977 and 1998–99 with estimates of respiratory and circulatory influenza-associated deaths ranging from about 5,000 to about 52,000, and an average of about 25,000. CDC believes that the range of deaths over the past 31 years (~3,000 to ~49,000) is a more accurate representation of the unpredictability and variability of flu-associated deaths.<ref>{{cite web |url=https://www.cdc.gov/flu/about/disease/us_flu-related_deaths.htm#how-many-die |title= Seasonal Influenza (Flu) – Questions and Answers Regarding Estimating Deaths from Influenza in the United States |publisher=Cdc.gov |access-date=23 October 2011 |url-status=live |archive-url=https://web.archive.org/web/20111016052954/http://www.cdc.gov/flu/about/disease/us_flu-related_deaths.htm#how-many-die |archive-date=16 October 2011 }}</ref> The annual toll from seasonal influenza in the US between 1979 and 2001 is estimated at 41,400 deaths on average.<ref name="Average Flu Deaths" /> Therefore, the H1N1 pandemic estimated mortality of 8,870 to 18,300 is just below the mid-range of estimates.<ref>{{cite web|url=http://flu.gov/individualfamily/about/h1n1/estimates_2009_h1n1.html#The%20Numbers |title=CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April – November 14, 2009 |publisher=Flu.gov |access-date=23 October 2011 |archive-url=https://web.archive.org/web/20111209165809/http://www.flu.gov/individualfamily/about/h1n1/estimates_2009_h1n1.html |archive-date=9 December 2011 }}</ref>

The 2009 pandemic caused US hospitals to make significant preparations in terms of hospital surge capacities, especially within the emergency department and among vulnerable populations. In many cases, hospitals were relatively successful in making sure that those patients most severely affected by the influenza strain were able to be seen, treated, and discharged in an efficient manner. A case-study of the preparation, planning, mitigation, and response efforts during the fall of 2009 is that of the Children's Hospital of Philadelphia (CHOP) which took several steps to increase the emergency department (ED) surge capacity response. CHOP used portions of the main lobby area as an ED waiting room; several of the region's hospital-based outpatient facilities were in use during evening and weekend hours for non-emergency cases; the ED's 24-hour short-stay unit was utilized to care for ED patients in a longer-term capacity; non-board certified physicians (in pediatric emergency medicine) and inpatient-unit medical nurses were utilized for ED patient care; hospital units normally utilized for other medical or therapeutic purposes were transformed into ED patient rooms; and rooms normally used for only one patient were expanded to at least a capacity of 2.<ref>{{cite journal | vauthors = Scarfone RJ, Coffin S, Fieldston ES, Falkowski G, Cooney MG, Grenfell S | title = Hospital-based pandemic influenza preparedness and response: strategies to increase surge capacity | journal = Pediatric Emergency Care | volume = 27 | issue = 6 | pages = 565–72 | date = June 2011 | pmid = 21642799 | doi = 10.1097/PEC.0b013e31821dc9d1 | s2cid = 11222604 }}</ref>

==Comparisons to other pandemics and epidemics==
Annual influenza epidemics are estimated to affect 5–15% of the global population. Although most cases are mild, these epidemics still cause severe illness in {{Nowrap|3–5 million}} people and 290,000–650,000 deaths worldwide every year.<ref name="WHO Influenza">{{cite web |url=https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal) |title=Influenza: Fact sheet |date=6 November 2018 |publisher=[[Organization:World Health Organization|World Health Organization]] (WHO) |access-date=25 January 2020 |archive-date=17 December 2019 |archive-url=https://web.archive.org/web/20191217001215/https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal) |url-status=live }}</ref> On average 41,400 people die of influenza-related illnesses each year in the United States, based on data collected between 1979 and 2001.<ref name="Average Flu Deaths">{{cite journal | vauthors = Dushoff J, Plotkin JB, Viboud C, Earn DJ, Simonsen L | title = Mortality due to influenza in the United States – an annualized regression approach using multiple-cause mortality data | journal = American Journal of Epidemiology | volume = 163 | issue = 2 | pages = 181–87 | date = January 2006 | pmid = 16319291 | doi = 10.1093/aje/kwj024 | quote = The regression model attributes an annual average of 41,400 (95% confidence interval: 27,100, 55,700) deaths to influenza over the period 1979–2001. | doi-access = free }}</ref> In industrialised countries, severe illness and deaths occur mainly in the high-risk populations of infants, the elderly and chronically ill patients,<ref name="WHO Influenza" /> although the H1N1 flu outbreak (like the 1918 Spanish flu) differs in its tendency to affect younger, healthier people.<ref name="Swine flu affect the young">{{cite web|url=http://bulletin.aarp.org/yourhealth/diseases/articles/swine_flus_tendency_to_strike_the_young_is_causing_confusion.html |title=Swine flu's tendency to strike the young is causing confusion |date=18 September 2009|work=Los Angeles Times | vauthors = Kaplan K |access-date=18 September 2009| archive-url= https://archive.today/20090919215923/http://bulletin.aarp.org/yourhealth/diseases/articles/swine_flus_tendency_to_strike_the_young_is_causing_confusion.html| archive-date= 19 September 2009 | url-status= live}}</ref>

In addition to these annual epidemics, Influenza A virus strains caused three global pandemics during the 20th century: the Spanish flu in 1918, Asian flu in 1957, and Hong Kong flu in 1968–69. These virus strains had undergone [[Biology:Antigenic shift|major genetic changes]] for which the population did not possess significant [[Medicine:Immunity (medical)|immunity]].<ref name="EID_Pandemics">{{cite journal | vauthors = Kilbourne ED | title = Influenza pandemics of the 20th century | journal = Emerging Infectious Diseases | volume = 12 | issue = 1 | pages = 9–14 | date = January 2006 | pmid = 16494710 | pmc = 3291411 | doi = 10.3201/eid1201.051254 }}</ref> Recent genetic analysis has revealed that three-quarters, or six out of the eight genetic segments, of the 2009 flu pandemic strain arose from the North American swine flu strains circulating since 1998, when a new strain was first identified on a factory farm in North Carolina, and which was the first-ever reported triple-hybrid flu virus.<ref>{{Cite news|url=http://www.hsus.org/farm/news/ournews/swine_flu_virus_origin_1998_042909.html |title=CDC Confirms Ties to Virus First Discovered in U.S. Pig Factories | vauthors = Greger M |date=26 August 2009 |publisher=The Humane Society of the United States |access-date=21 September 2009 |archive-url=https://web.archive.org/web/20090903111344/http://www.hsus.org/farm/news/ournews/swine_flu_virus_origin_1998_042909.html |archive-date=3 September 2009 }}</ref>

The Spanish flu began with a wave of mild cases in the spring, followed by more deadly waves in the autumn, eventually killing hundreds of thousands in the United States and 50–100 million worldwide.<ref>{{Cite news |url=https://www.usatoday.com/news/health/2009-05-26-swine-flu-decrease_N.htm |title=CDC expert says flu outbreak is dying down – for now | vauthors = Sternberg S |date=26 May 2009 |work=USA Today |access-date=17 September 2009 |url-status=live |archive-url= https://web.archive.org/web/20091007163933/http://www.usatoday.com/news/health/2009-05-26-swine-flu-decrease_N.htm |archive-date=7 October 2009 }}</ref> The great majority of deaths in the 1918 flu pandemic were the result of secondary bacterial pneumonia. The influenza virus damaged the lining of the bronchial tubes and lungs of patients, allowing common bacteria from the nose and throat to infect their lungs. Subsequent pandemics have had many fewer fatalities due to the development of antibiotic medicines which can treat pneumonia.<ref>{{cite journal | vauthors = Morens DM, Taubenberger JK, Fauci AS | title = Predominant role of bacterial pneumonia as a cause of death in pandemic influenza: implications for pandemic influenza preparedness | journal = The Journal of Infectious Diseases | volume = 198 | issue = 7 | pages = 962–70 | date = October 2008 | pmid = 18710327 | pmc = 2599911 | doi = 10.1086/591708 }}</ref>

The influenza virus has caused several pandemic threats over the past century, including the pseudo-pandemic of 1947 (thought of as mild because although globally distributed, it caused relatively few deaths),<ref name="EID_Pandemics" /> the [[Biology:1976 swine flu outbreak|1976 swine flu outbreak]] and the [[Biology:Influenza A virus subtype H1N1#1977 Russian flu|1977 Russian flu]], all caused by the H1N1 subtype.<ref name="EID_Pandemics"/> The world has been at an increased level of alert since the [[Unsolved:Severe acute respiratory syndrome|SARS]] epidemic in Southeast Asia (caused by the [[Biology:SARS coronavirus|SARS coronavirus]]).<ref>{{Cite news |date=29 April 2009 |vauthors=Brown D |title=System set up after SARS epidemic was slow to alert global authorities |newspaper=The Washington Post |url=https://www.washingtonpost.com/wp-dyn/content/article/2009/04/29/AR2009042904911.html |access-date=26 September 2009 |archive-date=7 May 2009 |archive-url=https://web.archive.org/web/20090507163851/http://www.washingtonpost.com/wp-dyn/content/article/2009/04/29/AR2009042904911.html |url-status=live }}</ref> The level of preparedness was further increased and sustained with the advent of the [[Biology:Influenza A virus subtype H5N1|H5N1 bird flu]] outbreaks because of H5N1's high fatality rate, although the strains currently prevalent have limited human-to-human transmission (anthroponotic) capability, or epidemicity.<ref>{{cite journal | vauthors = Beigel JH, Farrar J, Han AM, Hayden FG, Hyer R, de Jong MD, Lochindarat S, Nguyen TK, Nguyen TH, Tran TH, Nicoll A, Touch S, Yuen KY | display-authors = 6 | title = Avian influenza A (H5N1) infection in humans | journal = The New England Journal of Medicine | volume = 353 | issue = 13 | pages = 1374–85 | date = September 2005 | pmid = 16192482 | doi = 10.1056/NEJMra052211 | citeseerx = 10.1.1.730.7890 | collaboration = Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5 }}</ref>

People who contracted influenza before 1957 appeared to have some immunity to H1N1 flu. According to Daniel Jernigan, head of flu epidemiology for the U.S. CDC "Tests on blood serum from older people showed that they had antibodies that attacked the new virus&nbsp;... That does not mean that everyone over 52 is immune, since Americans and Mexicans older than that have died of the new flu".<ref name="Don McNeil, 20 May">{{Cite news |url=https://www.nytimes.com/2009/05/21/health/21swineflu.html |title=U.S. Says Older People Appear Safer From New Flu Strain |work=The New York Times | vauthors = McNeil Jr DG |date=20 May 2009 |access-date=26 September 2009 |url-status=live |archive-url=https://web.archive.org/web/20091007164253/http://www.nytimes.com/2009/05/21/health/21swineflu.html |archive-date=7 October 2009 }}</ref>

In June 2012, a model based study found that the number of deaths related to the H1N1 influenza may have been fifteen times higher than the reported laboratory confirmed deaths, with 80% of the respiratory and cardiovascular deaths in people younger than 65 years and 51% occurring in southeast Asia and Africa. A disproportionate number of pandemic deaths might have occurred in these regions and that efforts to prevent future influenza pandemics need to effectively target these regions.<ref>{{cite news | url = https://news.yahoo.com/h1n1-swine-flu-may-killed-15-times-more-202801465--abc-news-wellness.html | agency = ABC News Medical Unit | title = H1N1 Swine Flu May Have Killed 15 Times More Than First Said | date = 26 June 2012 | url-status = live | archive-url = https://web.archive.org/web/20160320120403/http://news.yahoo.com/h1n1-swine-flu-may-killed-15-times-more-202801465--abc-news-wellness.html | archive-date = 20 March 2016 | df = dmy-all }}, citing {{cite journal | vauthors = Dawood FS, Iuliano AD, Reed C, Meltzer MI, Shay DK, Cheng PY, Bandaranayake D, Breiman RF, Brooks WA, Buchy P, Feikin DR, Fowler KB, Gordon A, Hien NT, Horby P, Huang QS, Katz MA, Krishnan A, Lal R, Montgomery JM, Mølbak K, Pebody R, Presanis AM, Razuri H, Steens A, Tinoco YO, Wallinga J, Yu H, Vong S, Bresee J, Widdowson MA | display-authors = 6 | title = Estimated global mortality associated with the first 12 months of 2009 pandemic influenza A H1N1 virus circulation: a modelling study | journal = The Lancet. Infectious Diseases | volume = 12 | issue = 9 | pages = 687–95 | date = September 2012 | pmid = 22738893 | doi = 10.1016/S1473-3099(12)70121-4 | url = https://zenodo.org/record/1260250 | access-date = 6 September 2020 | archive-date = 5 March 2023 | archive-url = https://web.archive.org/web/20230305165544/https://zenodo.org/record/1260250 | url-status = live }}</ref>

A WHO-supported 2013 study estimated that the 2009 global pandemic respiratory mortality was ~10-fold higher than the World Health Organization's laboratory-confirmed mortality count (18.631). Although the pandemic mortality estimate was similar in magnitude to that of seasonal influenza, a marked shift toward mortality among persons less than 65 years of age occurred, so that many more life-years were lost. Between 123,000 and 203,000 pandemic respiratory deaths were estimated globally for the last nine months of 2009. The majority (62–85%) were attributed to persons under 65 years of age. The burden varied greatly among countries. There was an almost 20-fold higher mortality in some countries in the Americas than in Europe. The model attributed 148,000–249,000 respiratory deaths to influenza in an average pre-pandemic season, with only 19% in persons <65 years of age.<ref name="mortality estimate worldwide">{{cite journal | vauthors = Simonsen L, Spreeuwenberg P, Lustig R, Taylor RJ, Fleming DM, Kroneman M, Van Kerkhove MD, Mounts AW, Paget WJ | display-authors = 6 | title = Global mortality estimates for the 2009 Influenza Pandemic from the GLaMOR project: a modeling study | journal = PLOS Medicine | volume = 10 | issue = 11 | pages = e1001558 | date = November 2013 | pmid = 24302890 | pmc = 3841239 | doi = 10.1371/journal.pmed.1001558 | doi-access = free }}</ref>

The COVID-19 pandemic is not caused by an influenza virus but [[Biology:SARS-CoV-2|SARS-CoV-2]], a coronavirus which also primarily affects the respiratory system.

==See also==

* [[Medicine:SARS|SARS]]
* 2009 swine flu pandemic tables
* [[Medicine:2015–16 Zika virus epidemic|2015–16 Zika virus epidemic]]
* Black Death
* COVID-19 pandemic
* G4 EA H1N1
* Health crisis
* [[Medicine:MERS|MERS]]
* Public health emergency (United States)
* Western African Ebola virus epidemic

==References==
{{Reflist}}

==Further reading==
{{refbegin|30em}}
* {{cite journal | vauthors = Cannell JJ, Zasloff M, Garland CF, Scragg R, Giovannucci E | title = On the epidemiology of influenza | journal = Virology Journal | volume = 5 | issue = 1 | pages = 29 | date = February 2008 | pmid = 18298852 | pmc = 2279112 | doi = 10.1186/1743-422X-5-29 | doi-access = free }}
* {{cite book | vauthors = MacPhail T |date=2014 |title=The Viral Network |location=Ithaca; London |publisher=Cornell University Press |isbn=978-0-8014-7983-0 }}
* {{cite journal | vauthors = Smith GJ, Vijaykrishna D, Bahl J, Lycett SJ, Worobey M, Pybus OG, Ma SK, Cheung CL, Raghwani J, Bhatt S, Peiris JS, Guan Y, Rambaut A | display-authors = 6 | title = Origins and evolutionary genomics of the 2009 swine-origin H1N1 influenza A epidemic | journal = Nature | volume = 459 | issue = 7250 | pages = 1122–25 | date = June 2009 | pmid = 19516283 | doi = 10.1038/nature08182 | bibcode = 2009Natur.459.1122S | doi-access = free }}
* {{cite journal | vauthors = Soundararajan V, Tharakaraman K, Raman R, Raguram S, Shriver Z, Sasisekharan V, Sasisekharan R | title = Extrapolating from sequence – the 2009 H1N1 'swine' influenza virus | journal = Nature Biotechnology | volume = 27 | issue = 6 | pages = 510–13 | date = June 2009 | pmid = 19513050 | doi = 10.1038/nbt0609-510 | s2cid = 22710439 }}
* {{cite journal | title = Introduction and transmission of 2009 pandemic influenza A (H1N1) Virus – Kenya, June–July 2009 | journal = MMWR. Morbidity and Mortality Weekly Report | volume = 58 | issue = 41 | pages = 1143–46 | date = October 2009 | pmid = 19847148 | url = https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5841a1.htm | url-status = live | archive-url = https://web.archive.org/web/20110513193841/http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5841a1.htm | archive-date = 13 May 2011 | author1 = Centers for Disease Control Prevention (CDC) }}
{{refend}}

==External links==
<!-- Global first, then by continent, then by country. Lay first, more technical below that. -->
* {{Curlie|Health/Conditions_and_Diseases/Infectious_Diseases/Viral/Influenza/H1N1/|Influenza: H1N1}}
* [https://web.archive.org/web/20090427191833/http://www.who.int/csr/disease/swineflu/en/index.html Pandemic (H1N1) 2009] at the [[Organization:World Health Organization|World Health Organization]] (WHO)
* [https://web.archive.org/web/20090512121748/http://www.promedmail.org/pls/otn/f?p=2400:1000: International Society for Infectious Diseases] PROMED-mail news updates


{{Medical resources
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| MedlinePlus = 007421
| eMedicineSubj = article
| eMedicineTopic = 1673658
| MeshID = D053118
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{{2009 swine flu}}
{{Influenza}}

[[Category:Influenza A virus subtype H1N1]]

{{Sourceattribution|2009 swine flu pandemic}}