Influenza activity in Hong Kong during 2009–2011 and the sources of specimens for comparative analyses.

(A) Influenza virus isolation [20]; (B) Specimens included sera from a serial cross-sectional influenza serosurvey [3], EDTA-plasma harvested from the HKRCBTS blood donation archives, and sera from a longitudinal influenza serosurvey [20]. Color codes indicate the specimens used for the different comparative analyses in this study: green indicates 609 matched serum/EDTA-plasma specimens for antibody titer comparison; purple indicates 340 paired sera and 376 paired EDTA-plasma for estimating seroconversion rate against A/H3N2 during the 2010 season; and orange indicates 190 paired sera and 392 paired EDTA-plasma for estimating seroconversion rate against A/H1N1 during the 2011 season.