Level 2: Mid-level evidence

Influenza vaccination is associated with reduced rate of death or hospitalization for influenza or pneumonia in community-dwelling elderly persons, based on observational evidence (N Engl J Med. 2007;357:1373-1381). During 713,872 person-seasons of observation from 18 cohorts of patients older than 65 years in U.S. HMOs from 1990-2000, there were 4,599 hospitalizations for pneumonia or influenza and 8,796 deaths. Comparing vaccinated vs. unvaccinated persons per season, mean hospitalization rates (0.6% vs. 0.7%) and mean mortality (1% vs. 1.6%) were lower with vaccination. These differences were statistically significant after adjusting for other variables.