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Through July 2009, a total of 43,677 laboratory-confirmed cases of influenza A pandemic (H1N1) 2009 were reported in the United States, which is likely a substantial underestimate of the true number. Correcting for under-ascertainment using a multiplier model, we estimate that 1.8 million-5.7 million cases occurred, including 9,000-21,000 hospitalizations.
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Reed et al. (2009) studied this question.
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