Effectiveness and safety of pandemic influenza A (H1N1) 2009 vaccine in healthcare workers at a university hospital in Japan

SUMMARY: Pandemic influenza A (H1N1) virus (AH1pdm) emerged in April 2009. An inactivated, split-virus, unadjuvanted AH1pdm vaccine was manufactured in Japan, and vaccination was initiated with top priority for healthcare workers (HCWs) on October 19, 2009. A retrospective cohort study was conducted to evaluate the effectiveness of a single-dose vaccine for HCWs in a hospital in Japan. A total of 1,567 (84.5z) of 1,854 HCWs were vaccinated. Thirty-seven were infected with AH1pdm before the vaccine became available, and were excluded. The other 250 were not vaccinated for personal reasons. We analyzed the influenza infection rate with or without vaccination and related adverse events. Among the 1,817 HCWs without previous infection, 37 were infected with AH1pdm; 13 (5.2z) of 250 unvaccinated HCWs became infected, which was a significantly higher rate than the 24 (1.5z) of 1,567 vaccinated HCWs (P＝ 0.001). Multivariate analysis revealed that age of 20–29 years was a risk factor for infection (adjusted odds ratio [aOR], 3.7; P o 0.001), and that vaccination was a preventive factor (aOR, 0.20; P o 0.001). Adverse events occurred in 549 of 1,060 HCWs, but most were mild. Although vaccination was carried out during AH1pdm epidemic expansion, the single-dose AH1pdm vaccine proved effective in HCWs, and severe adverse events were rare.