Effectiveness of trivalent influenza vaccine among children in two consecutive seasons in a community in Japan.

Influenza vaccination is considered the single most important medical intervention for the prevention of influenza. The dose of trivalent influenza vaccine in children was increased almost double since 2011/12 season in Japan. We estimated the influenza vaccine effectiveness for children 1-11 years of age using rapid test kits in Isahaya City, involving 28,884 children-years, over two consecutive influenza seasons (2011/12 and 2012/13). Children were divided into two groups, vaccinated and unvaccinated, according to their vaccination record, which was obtained from an influenza registration program organized by the Isahaya Medical Association for all pediatric facilities in the city. There were 14,562 and 14,282 children aged from 1-11 years in the city in 2011 and 2012 respectively. In the 2011/12 season, the overall vaccine effectiveness in children from 1-11 years of age, against influenza A and B were 23% [95% confidence interval (CI): 14%-31%] and 20% [95% CI: 8%-31%], respectively. In the 2012/13 season, vaccine effectiveness against influenza A and B was 13% (95% CI: 4%-20%) and 9% (95% CI: -4%-21%), respectively. The vaccine effectiveness was estimated using the rapid diagnosis test kits. Age-stratified estimation showed that vaccine effectiveness was superior in younger children over both seasons and for both virus types. In conclusion, the trivalent influenza vaccine has a significant protective effect for children 1-11 years of age against influenza A and B infection in the 2011/12 season and against influenza A infection in the 2012/13 season in a community in Japan.