Serum was tested for antibodies to pandemic (H1N1) 2009 infl uenza virus by using the hemagglutination inhibition assay with A/California/7/2009 virus and turkey red blood cells (10) . A titer of <40 was defi ned as negative and >40 as positive. Nucleic acid detection was performed on nasal swabs by using reverse transcription PCR (RT-PCR) for infl uenza-specifi c and pandemic (H1N1) 2009 virus-specifi c sequences on swabs; kits were provided by the Centers for Disease Control and Prevention (Atlanta, GA, USA) (11) and an ABI-7500FAST instrument at the World Health Organization (WHO) Collaborating Centre for Reference and Research on Infl uenza in Melbourne.