Association between seasonal influenza vaccination in 2008-2009 and pandemic influenza A (H1N1) 2009 infection among school students from Kobe, Japan, April-June 2009.

We assessed the effect of seasonal trivalent inactivated influenza vaccination (TIV) on pandemic influenza 2009 (pH1N1)-related illness from April to June 2009 among 2849 students (aged 12-18 years). TIV was associated with an increase in the frequency of pH1N1-related illness among subjects (adjusted odds ratio, 1.47; 95% confidence interval, 1.14-1.89). TIV during the 2008-2009 season increased the risk of pH1N1-related illness from April to June 2009.