Efficacy of dual vaccination of pandemic H1N1 2009 influenza and seasonal influenza on institutionalized elderly: A one-year prospective cohort study

BACKGROUND: The influenza A (H1N1) 2009 pandemic was declared by the WHO in April 2009. In Hong Kong, the vaccination program began in December 2009 in addition to the annual seasonal trivalent influenza vaccination program. The clinical efficacy of dual vaccination was unknown. METHOD: From December 2009 to November 2010, a prospective 12-month cohort study on institutionalized elderly of nine nursing homes was conducted. Elderly persons who were followed up by the Hong Kong West Community Geriatric Assessment Team and had been vaccinated by the Department of Health were included. Outcome measures included all cause mortality, all cause hospitalization, hospitalization for fever on admission and hospitalization for pneumonia based on ICD-9-CM. RESULTS: 711 elderly persons were included. 274 received both seasonal influenza vaccine and (H1N1) 2009 vaccine (H1N1-TIV), 368 received seasonal influenza vaccine only (TIV alone) and 69 received no vaccination (unvaccinated). Baseline characteristics were well matched between the groups, except there were fewer females in the TIV alone. The 12-month mortality rates of the H1N1-TIV, TIV alone and unvaccinated were 10.6%, 19.8% and 29%, respectively. Multivariate analysis demonstrated that dual vaccination in the institutionalized elderly significantly reduced all cause mortality by 54% (Hazard Ratio [HR] 0.46; 95% confidence interval [CI] 0.29–0.72; p<0.001) and 74% (HR 0.26; CI 0.13–0.49; p<0.001), compared with vaccination of seasonal vaccination alone and no vaccination, respectively. Dual vaccination also reduced all cause hospitalization, hospitalization for fever on admission and hospitalization for pneumonia compared with seasonal vaccination alone and the unvaccinated group. CONCLUSION: Dual vaccination with both H1N1 and seasonal vaccinations provided additional protection to institutionalized elderly in reducing mortality and hospitalization.