Fig. 1

Antibody titers of A/California/07/2009 (2009 H1N1 pandemic influenza A) and A/Brisbane/59/2007 (H1N1 seasonal influenza A) viruses grouped by birth year before (from June, 2008 to April, 2009) and after the pandemic (from January to July, 2010) in Beijing, China. a, b Geometric mean titers (95% confidence intervals). c, d Percentage with microneutralization assay titers ≥40 (95% confidence intervals). Undetectable titer was set at the level of 10 for calculation of geometric mean titers. ≥60 years: equal and older than 60 years old; <60 years: younger than 60 years old. P value was calculated between people over and under 60 years old ※ before pandemic and ☆ after pandemic