Effectiveness of Influenza Vaccine in the Community-Dwelling Elderly

TLDR

Reliable estimates of influenza vaccine effectiveness in adults 65+ are essential for vaccination policy, yet short‑term studies and residual confounding can mislead past results. This study assessed long‑term influenza vaccine effectiveness in community‑dwelling seniors while explicitly addressing bias and residual confounding. Using pooled data from 20 HMO cohorts over ten seasons, logistic regression adjusted for covariates estimated vaccine effectiveness for hospitalization and death, with additional analyses probing bias and confounding across 713,872 person‑seasons. Vaccination was linked to a 27% reduction in pneumonia or influenza hospitalization (OR 0.73) and a 48% reduction in death (OR 0.52), with stable estimates across subgroups and sensitivity analyses confirming significant benefits, underscoring the need to improve vaccine delivery to this high‑priority group.

Abstract

Reliable estimates of the effectiveness of influenza vaccine among persons 65 years of age and older are important for informed vaccination policies and programs. Short-term studies may provide misleading pictures of long-term benefits, and residual confounding may have biased past results. This study examined the effectiveness of influenza vaccine in seniors over the long term while addressing potential bias and residual confounding in the results.Data were pooled from 18 cohorts of community-dwelling elderly members of one U.S. health maintenance organization (HMO) for 1990-1991 through 1999-2000 and of two other HMOs for 1996-1997 through 1999-2000. Logistic regression was used to estimate the effectiveness of the vaccine for the prevention of hospitalization for pneumonia or influenza and death after adjustment for important covariates. Additional analyses explored for evidence of bias and the potential effect of residual confounding.There were 713,872 person-seasons of observation. Most high-risk medical conditions that were measured were more prevalent among vaccinated than among unvaccinated persons. Vaccination was associated with a 27% reduction in the risk of hospitalization for pneumonia or influenza (adjusted odds ratio, 0.73; 95% confidence interval [CI], 0.68 to 0.77) and a 48% reduction in the risk of death (adjusted odds ratio, 0.52; 95% CI, 0.50 to 0.55). Estimates were generally stable across age and risk subgroups. In the sensitivity analyses, we modeled the effect of a hypothetical unmeasured confounder that would have caused overestimation of vaccine effectiveness in the main analysis; vaccination was still associated with statistically significant--though lower--reductions in the risks of both hospitalization and death.During 10 seasons, influenza vaccination was associated with significant reductions in the risk of hospitalization for pneumonia or influenza and in the risk of death among community-dwelling elderly persons. Vaccine delivery to this high-priority group should be improved.