Relationship between elderly influenza and childhood vaccination coverage: The effect of urbanization and income inequality

A growing body of evidence suggests that vaccination of children reduces the spread of pneumonia and influenza (P&I) to the elderly. However, vaccination coverage in children and influenza-related outcomes in the elderly vary by socioeconomic status and related demographic characteristics. The purpose of this study is to examine two aggregate-level sociodemographic factors—urbanicity and income inequality—as potential modifiers of the relationship between vaccination coverage in children and P&I in the elderly. Using approximately 14 million Medicare hospitalization records from 1991 to 2004, we estimated the increase in P&I hospitalization rates by age for each influenza season (July-June) and state. Vaccination coverage in children and the elderly and sociodemographic characteristics were abstracted from the National Immunization Survey and the US Census Bureau. Longitudinal mixed effects models were used to model these potential associations. Overall, each five-percentage point increase in child vaccination coverage was associated with a decrease in elderly P&I hospitalizations of 6.9% (p<0.001). This was also associated with a decrease in P&I hospitalizations in the elderly of 6.3% in states in the highest tertiles of income inequality, and 8.6% and 7.5% in states in the highest and lowest tertiles of urbanicity, respectively. These results suggest that vaccination of children may be a useful strategy to control influenza and pneumonia in the elderly. However, given an insufficient supply for universal vaccination, an effective strategy could target more urbanized and rural areas and those with high income inequality, in order to achieve the greatest benefit for the elderly community.