We conducted 2 separate community-based studies of influenza virus infection in Hong Kong with broadly similar protocols for recruitment and follow-up. In both studies, outpatients with recent-onset acute respiratory illness who presented within 48 hours of symptom onset were recruited; of individuals who provided informed consent, those with a positive result on a QuickVue Influenza A1B rapid diagnostic test (Quidel Corp) were invited to continue with follow-up. In one prospective, multicenter study (Influenza Resistance Information Study [IRIS] ), patients recruited between 20 January 2010 and 24 November 2010 were followed up to examine natural prevalence and/or emergence of resistance to antivirals among circulating influenza virus strains. In a separate household transmission study (HTS), eligible patients were recruited between 9 January 2008 and 29 September 2008 and followed up as part of a study investigating the effectiveness of nonpharmacological interventions [1] .