Measles vaccine efficacy in children previously vaccinated at 12 months of age

During a large outbreak of measles in Ohio in 1976 it was possible to measure measles vaccine efficacy by age at time of vaccination and number of years since vaccination. Using a summed incidence method to control for the confounding variable introduced by mass immunization clinics held during the outbreak, vaccine efficacy was greater than 95% for children vaccinated at 12, 13, and 14 or more months of age. Vaccine efficacy for those vaccinated at 12 months of age was notably better than for those vaccinated at younger ages but not different from those vaccinated at older ages. Although recently administered vaccine appeared more efficacious than vaccine administered in the past, this difference was not significant when controlled for age at vaccination. Evaluation of the mass clinics held during the outbreak demonstrated that 59.6% of the inadequately immunized children attended the clinics, but this was not substantially different from the proportion of adequately immunized who attended (52.4%). Recommendations for measles revaccination need not include children previously vaccinated at 12 months of age or greater.