Questions have been raised regarding the apparent lower than expected impact on excess mortality rates of increasing influenza vaccination rates among elderly persons in the United States. One possible explanation might be that influenza vaccine is less effective among elderly persons. To explore whether age is independently associated with influenza vaccine effectiveness, we used computerized administrative and clinical data to study >120,000 elderly members from three large managed care organizations in the United States over each of four influenza seasons, 1996–1997 through 1999–2000. After multivariable logistic regression to control for covariates and potential confounders, influenza vaccine effectiveness was similar among persons ≥85 years when compared with persons 65–74 years of age. However, persons ≥85 years were more likely to have medical comorbidities and a higher risk of dying. Unvaccinated persons ≥85 years with underlying chronic medical conditions were nearly 25 times more likely to die than were healthy, vaccinated persons 65–74 years of age. The impact of an aging population that also develops more chronic medical conditions on the absolute numbers of excess deaths due to influenza should be taken into account when attempting to evaluate the population level impact of increasing vaccination rates. Our data suggest that influenza vaccines themselves are otherwise similarly effective among the very old as among younger elderly persons after taking into account both age and underlying burden of illness.
- Vaccine efficacy