Association of livebirths and the resurgence of measles

Background. The basis for the resurgence of measles in the US in 1989 and 1990 is not understood. This analysis was undertaken to test the hypothesis that an increase in the number of livebirths was associated with the resurgence of measles in the US. Methods. We undertook an ecologic analysis of 20 cities/counties in the US with documented rates of immunization among 2-year-old children. Results. Over the 6-year period 1985-1990, the numbers of livebirths and of susceptible preschool aged children increased by 18.5% and 17.7%, respectively. Livebirths, and the number and density of susceptible preschool-age children were significantly associated with the number and incidence of measles among preschool children (r = 0.83, P = 0.04). In a comparison between counties, numbers of livebirths were also significantly correlated with the mean number (r = 0.73, P = 0.0003) and incidence of measles cases (r = 0.51, P = 0.02). Mean immunization rates of 2-year-old children were also associated with the mean incidence of measles (r = -0.66, P = 0.0015, and r = -0.57, P = 0.009, respectively). In a logistic regression model, levels of immunization and susceptible density were independent predictors of measles epidemics among preschool children. Conclusions. These data suggest that the increase in livebirths, leading to an increase in the number and density of susceptible hosts, was associated with the resurgence of measles among preschool-age children.