Inactivated influenza vaccine effectiveness against influenza-like illness among young children in Japan--with special reference to minimizing outcome misclassification.

The aim of the present study was to investigate the influenza vaccine effectiveness among young children in Japan. Study subjects were recruited from 43 pediatric clinics. Influenza-like illness (ILI) was defined as an acute febrile illness with respiratory symptoms; ILI with a fever of > or =39 degrees C was considered to be severe ILI (SILI). The adjusted OR of vaccination significantly decreased to 0.75 for SILI. Influenza vaccination for young children had a protective effect on the occurrences of SILI. This study also indicated that three key tools (case surveillance with equal scrutiny, confining observation to the peak epidemic period, and adoption of strict criteria for ILI) could minimize outcome misclassification and thus provide adequate methodology for monitoring vaccine effectiveness without laboratory confirmation.