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Most viral infections — including influenza — cause symptoms in only a fraction of infected individuals, but those with subclinical infection might also transmit the disease. Knowing the proportion of such individuals in a cohort is essential for epidemiologic predictions.
To estimate the burden and severity of influenza during recent years in England, researchers conducted a household-level community cohort study of acute respiratory illnesses during the 2006–2007, 2007–2008, and 2008–2009 periods of seasonal influenza circulation and the first (spring and summer, 2009), second (autumn and winter, 2009), and third (winter, 2010–2011) waves of the 2009 H1N1 influenza pandemic. Participants reported their health status weekly and, if they exp…