Importance of catch-up vaccination: experience from a varicella outbreak, Maine, 2002-2003.

Objective: During December 2002 to January 2003, a varicella outbreak occurred in an elementary school in Maine. Just 1 month before detecting the outbreak, Maine implemented varicella vaccine requirements for child care but did not require vaccination for school entry. We investigated this outbreak to examine reasons for its occurrence, including vaccine failure.

Methods: A self-administered questionnaire was sent to all students' parents to determine student disease status, medical conditions, and vaccination status, which was further confirmed by review of medical records. Parental reporting of chickenpox/varicella that occurred since September 1, 2002, in a student who attended the school was used to define a case. Parents of cases were interviewed by telephone about disease characteristics. Disease severity was classified on the basis of the number of skin lesions and the occurrence of complications. Vaccine effectiveness was calculated by comparing varicella attack rates for any disease, for moderate to severe disease, and for severe disease among vaccinated and unvaccinated students.

Results: We obtained complete information for 296 (81%) of 364 students. Varicella vaccine coverage was 74% overall and decreased by grade, from 90% in kindergarten to 60% in third grade. Attack rates increased significantly from 14% in kindergarten to 37% in third grade. Of the 53 varicella cases, 36 (68%) were unvaccinated, 12 (22%) were vaccinated, and 5 (10%) had previous disease history. Vaccine effectiveness was 89% (95% confidence interval [CI]: 79-94%) against disease of any severity, 96% (95% CI: 88-99%) against moderate to severe disease, and 100% (95% CI: undefined) against severe disease. Twenty-two percent of unvaccinated students had severe disease and 1 was hospitalized for a skin infection, whereas none of the vaccinated cases reported severe disease.

Conclusion: This outbreak was attributable primarily to failure to vaccinate, especially among children in grades 1 through 3. Catch-up vaccination of susceptible older children and adolescents is especially important to prevent accumulation of susceptibility in these groups, in which the natural disease is more severe. School entry requirements will contribute to a more rapid implementation of the existing recommendations for vaccination.