The Impact of Influenza Vaccinations on the Adverse Effects and Hospitalization Rate in the Elderly: A National Based Study in an Asian Country

OBJECTIVES To examine the risk of adverse effects of special interest in persons vaccinated against seasonal influenza compared with unvaccinated persons aged 65 and above. METHODS We retrospectively observed 41,986 vaccinated elderly persons and 50,973 unvaccinated elderly persons in Taiwan from October 1, 2008, through September 30, 2009, using the National Health Insurance database. Neurological and autoimmune disorders and one-year hospitalization rates and in-hospital mortality rates were analyzed according to the vaccination status. Propensity score analysis was used to assess the relationship between adverse outcomes, hospitalization rates, and vaccination status. RESULTS 45% of the elderly received influenza vaccination. Multiple logistic regression showed that the probability of being vaccinated was related to more patients visiting for URI symptoms (odds ratio (OR), 1.03; 95% CI, 1.02-1.03), men (OR, 1.15; 95% CI, 1.12-1.17), increased age (OR, 1.02; 95% CI, 1.02-1.03), and more comorbidities (OR, 1.2; 95% CI, 1.17-1.23). There were no statistical differences in neurological and autoimmune diseases between the vaccinated and unvaccinated individuals using propensity score analysis, but vaccinated persons had a reduced hospitalization rate of 19% (odds ratio [OR], 0.81; 95% CI, 0.77-0.84) for the first six-months and 13% for one-year of follow-up (OR, 0.87; 95% CI, 0.85-0.9). CONCLUSIONS Based on data from the one-year follow-ups among 93,049 elderly persons in Taiwan, reassuring results for selected neurological and autoimmune diseases were found among the vaccinated individuals after adjusting other factors. Influenza vaccination decreased the risk for hospitalization. Public health strategies must continue to improve the influenza vaccination rate among the elderly with information based upon tangible evidence.