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©2013 UpToDate ® Print Email Antigenic subtypes of influenza A virus associated with pandemics or epidemics Year Subtype Severity of outbreak 1889-90 H2N8* Severe pandemic 1900-03 H3N8* Moderate epidemic, probable 1918-19 H1N1, formerly HswN1 • Severe pandemic 1933-35 H1N1, formerly H0N1 • Mild epidemic 1946-47 H1N1 Mild epidemic 1957-58 H2N2 Severe pandemic 1968-69 H3N2 Moderate pandemic 1977-78 H1N1 Mild pandemic 2009-10 H1N1 Mild to moderate pandemic Δ * As determined by retrospective serologic survey of individuals alive during those years. • Hemagglutinins formerly designated as Hsw and H0 are now classified as variants of H1. Δ During the 2009-2010 H1N1 influenza A pandemic, mortality was relatively high among individuals <65 years of age and relatively low among those ≥65 years of age. (Armstrong GL, Brammer L, Finelli L. Timely assessment of the severity of the 2009 H1N1 influenza pandemic. Clin Infect Dis 2011; 52 [Suppl 1]:S83.) Data adapted from: Dolin R. Influenza. In: Harrison's Principles of Internal Medicine, 17th ed, Braunwald E, Fauci AS, Kasper DL, et al (Eds), McGraw Hill Book Company. New York 2008. p.1127.