Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009

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.