Proportion of sequential serum samples obtained in different sampling time from the 306-subjects cohort with haemagglutination inhibition titre above the cutoff.

*1st sampling time refer to samples collected in 2008 (after 08–09 seasonal influenza vaccination), 2nd sampling time refer to samples collected during April to June in 2009 (after the 08–09 influenza season) and 3rd sampling time refer to samples collected during September to October in 2009.†Among those samples with HI titer≥40, 14 (9.8%), 12 (8.2%), and 3 (18.8%) showed neutralization antibody titer>10 in the 5–18, 19–60, and >60 age groups, respectively.