Influenza vaccination in children reduces influenza-associated hospitalizations in seniors, 2002-06

A growing body of evidence suggests that children play an integral role in the transmission of influenza, not only to other children, but also to seniors, who are especially vulnerable to the effects of influenza--namely morbidity and mortality. In this study, we build upon prior research of pneumonia vaccination in children and hypothesize that, in the absence of universal vaccination, vaccinating children against influenza would have a beneficial effect on reducing pneumonia and influenza-related (P&I) hospitalizations in seniors (≥65). We abstracted approximately 5 million hospitalization records from the Center for Medicare and Medicaid Services for four influenza seasons, 2002-2006. State-level influenza annual vaccination coverage in children was obtained from the National Immunization Survey, and influenza vaccination coverage in the elderly was obtained from the Behavioral Risk Factor Surveillance System. We estimated a single year age distribution of rates of P&I hospitalization by state for each influenza season. We observed an exponential acceleration in the P&I rates with age for each influenza season. State- and season-specific acceleration rates were regressed against the percentage of vaccinated children or seniors, or both. Child influenza vaccination coverage was negatively associated with elderly P&I hospitalizations for each season, and was strongest in the 2003-04 season (b = -0.096, p = 0.027). Influenza vaccination coverage of the elderly was not significantly associated with the acceleration in P&I rates in any of the six seasons in any of the models. These results suggest that vaccination of children against influenza induced herd immunity against influenza for seniors.