Vaccine Effectiveness During a Varicella Outbreak Among Schoolchildren: Utah, 2002–2003

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Abstract
Objectives. In the context of a chickenpox outbreak involving 2 Utah elementary schools, we conducted an investigation to assess vaccine effectiveness, describe illness severity, and examine risk factors for breakthrough varicella (ie, varicella in those who have been vaccinated). Methods. All parents were asked to complete a questionnaire about their child’s medical history. Parents of children with recent varicella were interviewed, and vaccination records were verified. Lesions were submitted for polymerase chain reaction testing. Results. Questionnaires were returned for 558 (93%) of 597 students in school A and 924 (97%) of 952 students in school B. A total of 83 schoolchildren (57 unvaccinated and 26 vaccinated) had varicella during the October 2002 through February 2003 outbreak period. An additional 17 cases occurred among household contacts, including infants and adults. Polymerase chain reaction analysis recovered wild-type varicella. Vaccine effectiveness was 87%. With 1 notable exception, vaccinated children tended to have milder illness. Risk factors for breakthrough varicella included eczema, vaccination ≥5 years before the outbreak, and vaccination at ≤18 months of age. Restricting analysis to children vaccinated ≥5 years before the outbreak, those vaccinated at ≤18 months of age were more likely to develop breakthrough varicella (relative risk: 9.3; 95% confidence interval: 1.3–68.9). Conclusions. The vaccine, administered by >100 health care providers to 571 children during a 7-year time period, was effective. Risk factors for breakthrough varicella suggest some degree of biological interaction between age at vaccination and time since vaccination.