Possible Herd Immunity in the Elderly Following the Vaccination of School Children with Live, Attenuated Trivalent Influenza Vaccine: A Person-Level Analysis

Models predict that immunizing as few as 20% of school children, important transmitters of influenza, will reduce influenza-related illness in the elderly. We evaluated the potential herd immunity during three influenza seasons, 2005-2006, 2006-2007 and 2007-2008, which followed the immunization of >= 40% of school children in Knox County (KC), TN, with live, attenuated influenza vaccine. Individual-level demographic, health status and health service utilization information about KC residents >= 65 years and those residing in the 8 surrounding counties was obtained from the United States Medicare Program's administrative data. Influenza seasons were identified based on virus isolation. Pneumonia and influenza (P&I) hospitalization rates per 1,000 were compared between the elderly residing in the two areas for the three influenza seasons, and the 3 prior seasons. Differences-in-difference multivariate analysis allowed us to estimate the effect of the school-based immunization program on P&I hospitalization rates simultaneously adjusting for other important individual-level covariates. The age-adjusted rates among the KC residents were significantly lower, 4.62 and 6.02 versus 6.54 and 7.58 than in the residents of the comparison counties during the first two intervention seasons, p = 0.001 and 0.037, respectively, but not in the third. However, after adjusting for the traditionally lower rates of P&I hospitalization in the comparison counties, as well as for the other covariates, we were not able to demonstrate a statistically significant effect of the vaccination program in reducing the rates in either group of the elderly. The impact of the covariates was as expected. Those associated with increased P&I hospitalization rates were increasing age, lower income, poorer health status, prior hospitalization (particularly for P&I), and high prior use of physician services. Influenza immunization of an elderly person reduced his/her probability of being hospitalized for P&I. In conclusion, Immunization of >= 40% of school children did not result in a reduction of P&I hospitalization rates among the elderly. We believe that the failure to show an impact was likely due to the high level of immunization among the elderly (>60%). Administration of influenza vaccine to children as a way to protect the elderly in situations where vaccine supplies are limited or the elderly are not accustomed to receiving influenza vaccine may still be appropriate. (C) 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of Prof. Ray Spier