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The 2009 pandemic influenza A(H1N1) virus has a higher incidence in children and young adults, a pattern that has also been reported in seasonal influenza caused by the influenza A(H1N1) virus. We analysed age at infection in symptomatic patients with influenza in the Basque Country (northern Spain), reported through the sentinel influenza surveillance system which monitors 2.2-2.5% of the population. Between September 1999 and August 2009, influenza A(H3N2) or seasonal influenza A(H1N1) was detected in 941 patients, and from April to August 2009, pandemic influenza A(H1N1) was detected in 112 patients. The H3/H1 seasonal influenza ratio was between 3.3 and 3.4 in the under 60 year-olds, but 9.8 in older individuals, suggesting that people born before 1950 have residual immunity against the influenza A H1N1 subtype (both seasonal and pandemic). Introduction In 1957, the Asian influenza pandemic was caused by influenza A(H2N2) virus, which circulated until 1968 when it was displaced by the influenza A(H3N2) virus which was responsible for the Hong Kong pandemic. Before 1957, direct descendants of the influenza A(H1N1) virus that had caused the 1918 pandemic (Spanish flu) had circulated. In 1977, an influenza A(H1N1) strain re-emerged, which, together with the dominant influenza A(H3N2) strain, has been the cause of seasonal human influenza for more than three decades [1]. Despite the prolonged co-circulation of both subtypes, few studies have analysed their ability to affect distinct age groups. The current pandemic influenza A(H1N1) virus, influenza A(H1N1)v, which emerged in the spring of 2009, has spread throughout the world. The aim of this study was to compare the distribution in distinct age groups of infections caused by the two subtypes of seasonal influenza in the past 10 seasons and refer therelate this to recent infections due to influenza A(H1N1)v.


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