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

Abstract

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 ≤15 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 ≥15 months of age. © 2002 Infectious Diseases Society of America.