Our findings have important limitations. A minority of all patients had respiratory specimens tested by RT-PCR, a large number of patients who were triaged were not confirmed with seasonal influenza or 2009 pandemic influenza A (H1N1) virus infection. No testing for other etiologies of acute respiratory illness was performed. Oseltamivir treatment among hospitalized patients was not randomized among cases and control. No comparison group was available to assess oseltamivir effectiveness for the treatment of 2009 pandemic influenza A (H1N1).