The Effectiveness of Influenza Vaccine Against Influenza A (H3N2) Virus Infections in Nursing Homes in Niigata, Japan, During the 1998-1999 and 1999-2000 Seasons

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Abstract
Objective: : To evaluate the effectiveness of influenza vaccines against influenza-like illness (ILI) among nursing home residents.Design: : Prospective, nonrandomized, cohort study.Setting: : Nine nursing homes during the 1998-1999 influenza season and 11 nursing homes during the 1999-2000 influenza season in Niigata Prefecture, Japan.Participants: : Six hundred ninety-nine residents and 440 healthcare workers (HCWs) during the first season, and 930 residents and 517 HCWs during the second season, with vaccination rates ranging from 0% to 97.7%.Results: : Overall, ILI decreased from 24.3% during the 1998-1999 season to 8.8% during the 1999-2000 season. Multivariate analysis adjusted for several factors, including gender, age, underlying diseases, and resident and HCW vaccination rates, failed to demonstrate clear individual protection of residents (relative risk [RR], 1.42; P = .2 for the first season; RR, 0.95; P = .9 for the second season). However, vaccination rates of 60% or greater for residents and HCWs reduced the risk of ILI, and also could prevent outbreaks during the 2 seasons. Highly impaired activities of daily living and chronic respiratory diseases were significantly associated with increased ILI.Conclusions: : A high vaccination rate for both residents and HCWs may reduce the risk of ILI and institutional outbreaks in nursing homes