The 2009 swine flu pandemic, caused by the H1N1 influenza virus and declared by the World Health Organization (WHO) from June 2009 to August 2010, is 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). The first two cases were discovered independently in the United States in April 2009. The virus appeared to be a new strain of H1N1 that resulted from a previous triple reassortment of bird, swine, and human flu viruses which further combined with a Eurasian pig flu virus, leading to the term "swine flu".
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. The lower value of 700 million is more than the 500 million people estimated to have been infected by the Spanish flu pandemic. However, the Spanish flu infected approximately a third of the world population at the time, a much higher proportion.
The number of lab-confirmed deaths reported to the WHO is 18,449 and is widely considered a gross underestimate. 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. 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 seasonal flu. For comparison, the WHO estimates that 250,000 to 500,000 people die of seasonal flu annually.
^Perez-Padilla R, de la Rosa-Zamboni D, Ponce de Leon S, Hernandez M, Quiñones-Falconi F, Bautista E, et al. (August 2009). "Pneumonia and respiratory failure from swine-origin influenza A (H1N1) in Mexico". The New England Journal of Medicine. 361 (7): 680–89. doi:10.1056/NEJMoa0904252. PMID19564631.
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^Bautista E, Chotpitayasunondh T, Gao Z, Harper SA, Shaw M, Uyeki TM, et al. (May 2010). "Clinical aspects of pandemic 2009 influenza A (H1N1) virus infection". The New England Journal of Medicine. 362 (18): 1708–19. doi:10.1056/NEJMra1000449. hdl:2381/15212. PMID20445182.
^Jain S, Kamimoto L, Bramley AM, Schmitz AM, Benoit SR, Louie J, et al. (November 2009). "Hospitalized patients with 2009 H1N1 influenza in the United States, April–June 2009". The New England Journal of Medicine. 361 (20): 1935–44. CiteSeerX10.1.1.183.7888. doi:10.1056/NEJMoa0906695. PMID19815859. 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 1May to 9June 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.
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