Outbreak of Varicella at a Day-Care Center despite Vaccination

Background In seven studies of the effectiveness of the varicella vaccine conducted since it was licensed, the effectiveness was 71 to 100 percent against disease of any severity and 95 to 100 percent against moderate and severe disease. We investigated an outbreak of varicella in a population of children with a high proportion of vaccinees who were attending a day-care center in a small community in New Hampshire. Methods Using standardized questionnaires, we collected information about the children's medical and vaccination history from parents and health care providers. The analysis of the effectiveness of the vaccine and of risk factors for vaccine failure was restricted to children who were enrolled in the day-care center continuously during the outbreak and attended for one week or more and who were cared for in the building that represented the epicenter of the outbreak, since transmission was not documented in a second building. Results Varicella developed in 25 of 88 children (28.4 percent) betwe...