Herd immunity in adults against influenza-related illnesses with use of the trivalent-live attenuated influenza vaccine (CAIV-T) in children

Highest attack rates for influenza occur in children. Immunization of schoolchildren with inactivated influenza vaccine in Michigan and Japan was associated with decreased morbidity and mortality, respectively, in older community contacts. An open-labeled, non-randomized, community-based trial in children with the cold adapted influenza vaccine, trivalent (CAIV-T) was initiated to determine the coverage necessary to reduce spread of influenza in the community. Age-specific baseline rates of medically attended acute respiratory illness (MAARI) for Scott and White Health Plan (SWHP) members at intervention (Temple and Belton) and comparison communities (Waco, Bryan, and College Station) were obtained in 1997-1998. During three subsequent vaccination years, 4298, 5251 and 5150 children received one dose per season of CAIV-T. Vaccinees represented 20-25% of the age-eligible children. Age-specific MAARI rates were compared for SWHP members in the intervention and comparison sites during the influenza outbreaks. Baseline age-specific MAARI rates per 100 persons for the influenza season were comparable between the intervention and comparison communities. In the subsequent three influenza seasons, the age groups 35-44, 45-54, 55-65 and >64 years experienced reductions in MAARI rates in the intervention communities. In adults ≥35 years of age, significant reductions in MAARI of 0.08 (95% CI: 0.04, 0.13), 0.18 (95% CI: 0.14, 0.22) and 0.15 (95% CI: 0.12, 0.19), were observed in the influenza seasons for vaccination years 1, 2 and 3, respectively. No consistent reduction in MAARI rates was detected in the younger age groups. Vaccination of approximately 20-25% of children, 1.5-18 years of age in the intervention communities resulted in an indirect protection of 8-18% against MAARI in adults ≥35 years of age.