An outbreak of varicella among children attending preschool and elementary school in Illinois

In our investigation of a varicella outbreak among students in preschool, kindergarten, and grades 1-3 in Winnebago County, Illinois, we found an overall varicella vaccine efficacy of 88%, evidence that the circulating virus was a wild-type strain (as determined by polymerase chain reaction analysis), and evidence that vaccination of children less than or equal to15 months of age was associated with an increased risk for breakthrough varicella (relative risk, 3.7; 95% confidence interval, 1.1-13.1; P=.04). The efficacy of varicella vaccine might be improved if administration of the initial vaccine dose is delayed until children are greater than or equal to15 months of age.