Benefits of influenza vaccination on influenza-related mortality among elderly in the US: an unexpected finding

Although elderly influenza vaccination coverage increased from ∼15% to ∼65% during 1980–1999 in the US, estimates of influenza-related mortality also increased during this period. We examined these apparently conflicting findings by adjusting mortality estimates for aging within the elderly and the incidence of influenza A (H3N2) virus circulation. Using national mortality statistics for 1968 through 1999, we generated age-specific monthly rates for pneumonia and influenza (P&I) and all-cause mortality for persons ≥65 years of age. We estimated influenza-related mortality as the winter excess in mortality over a Serfling model baseline. After adjusting for age and considering only A (H3N2)-dominated seasons, we found that excess mortality declined sharply among younger elderly (65–74 years) during 1968–1980, but remained level after 1980. Among the most elderly (85+ years), excess mortality rates were essentially unchanged over the entire study period. In conclusion, the increase in elderly influenza vaccination coverage in the US after 1980 was not accompanied by a decline in influenza-related mortality. We hypothesize that disparity in vaccination rates among frail elderly, combined with reduced responsiveness to vaccination with age, may account for these findings.