Age-specific incidence of chickenpox.

Because licensure of a chickenpox (varicella) vaccine is likely soon, it is important to ascertain the age-specific incidence of chickenpox. Increasing vaccine coverage and a resulting decrease in transmission may result in an accumulation of susceptible adults, followed by a shift of incidence into those older age groups in future years. Valid baseline age-specific incidence will make it possible to detect this phenomenon. Two studies were conducted in Kentucky to assess age-specific incidence of chickenpox. The first assessed chickenpox occurrence in two consecutive school-year cohorts of children from a geographically representative sample of Kentucky primary schools. The second gathered information from household members of those persons interviewed in the Behavioral Risk Factor Surveillance System telephone survey. The age-specific rates are remarkably similar between studies. Rates peak during the preschool and kindergarten years (ages 3-6). Approximately 20 percent of children remain susceptible to chickenpox after age 8 in both studies. The results from these two surveys will be valuable baselines for comparison with findings in incidence studies that will be performed after vaccine licensure.