Association of Influenza Vaccination Coverage in Younger Adults With Influenza-Related Illness in the Elderly

Background: Older adults have the highest influenza-related morbidity and mortality risk, but the influenza vaccine is less effective in the elderly. It is unknown whether influenza vaccination of nonelderly adults confers additional disease protection on the elderly population.

Methods: We examined the association between county-wide influenza vaccination coverage among 520 229 younger adults (aged 18-64 years) in the Behavioral Risk Factors Surveillance System Survey and illnesses related to influenza in 3 317 709 elderly Medicare beneficiaries aged ≥65 years, between 2002 and 2010 (13 267 786 person-years). Results were stratified by documented receipt of a seasonal influenza vaccine in each Medicare beneficiary.

Results: Increases in county-wide vaccine coverage among younger adults were associated with lower adjusted odds of illnesses related to influenza in the elderly. Compared with elderly residents of counties with ≤15% of younger adults vaccinated, the adjusted odds ratio for a principal diagnosis of influenza among elderly residents was 0.91 (95% confidence interval, .88-.94) for counties with 16%-20% of younger adults vaccinated, 0.87 (.84-.90) for counties with 21%-25% vaccinated, 0.80 (.77-.83) for counties with 26%-30% vaccinated, and 0.79 (.76-.83) for counties with ≥31% vaccinated (P for trend <.001). Stronger associations were observed among vaccinated elderly adults, in peak months of influenza season, in more severe influenza seasons, in influenza seasons with greater antigenic match to influenza vaccine, and for more specific definitions of influenza-related illness.

Conclusions: In a large, nationwide sample of Medicare beneficiaries, influenza vaccination among adults aged 18-64 years was inversely associated with illnesses related to influenza in the elderly.