A prospective, Internet-based study of the effectiveness and safety of influenza vaccination in the 2001-2002 influenza season

The effectiveness of the influenza vaccine used in the 2001-2002 influenza season in Japan was investigated in a large-scale, geographically widely distributed, Internet-based study. Data were collected from 8841 of 9902 subjects registered by 38 clinics prior to the start of influenza season. Subjects were categorized into three groups by vaccination regimen: unvaccinated, vaccinated once, and vaccinated twice. Efficacy was also analyzed for three age groups: 0-15, 16-64, and 65-104 years. Influenza-like illness (ILI) was diagnosed according to Ministry of Health (MWH, Labor and Welfare in Japan) criteria. Laboratory-confirmed influenza cases were analyzed separately. The respective vaccine efficacy in the 0-15 years group for the one- and two-dose regimens was 67.6 and 84.5% for ILI and 54.0 and 79.8% for laboratory-confirmed influenza. Influenza vaccination was also shown to be effective in subjects 16-64 years. Vaccine effectiveness was not able to be determined for the over 65 years group, probably due to an insufficient number of infected patients. These results suggest that influenza vaccination is effective for children and adults and that a two-dose regimen is superior to a single dose in children 0-15 years.