Postlicensure effectiveness of varicella vaccine during an outbreak in a child care center

H. S. Izurieta, P. M. Strebel and P. A. Blake
Division of Public Health, Georgia Department of Human Resources, Atlanta, USA.

CONTEXT: Because lyophilized varicella vaccine must be stored frozen at -15 degrees C or less (> or = 5 degrees F) and administered within 30 minutes after reconstitution, the potential exists for decreased vaccine effectiveness when the vaccine is used under field conditions. OBJECTIVES: To describe an outbreak of varicella in a child care center and to determine postlicensure effectiveness of varicella vaccine. DESIGN: Retrospective cohort study. SETTING: A child care center in DeKalb County, Georgia, in 1996. PARTICIPANTS: Of the 184 children registered in the child care center, 148 were eligible for the study based on absence of history of varicella before January 1, 1996. MAIN OUTCOME MEASURES: Data on disease status, severity and impact of disease, and risk factors for varicella and for vaccine failure were obtained from parents and their children's pediatricians. Varicella vaccine effectiveness was calculated among children aged 12 months or older (eligible for vaccination) using the cohort method. RESULTS: The outbreak started on January 17, 1996, and lasted 15 weeks. Of the 148 eligible children, 81 (55%) developed varicella. Cases among children younger than 12 months (n =7) were more severe than cases among older children. Varicella occurred in 9 (14%) of 66 vaccinated children and 72 (88%) of 82 unvaccinated children. Varicella was less severe and resulted in fewer days of absence from the child care center among vaccinated compared with unvaccinated cases. Varicella vaccine effectiveness against all forms of disease was 86% (95% confidence interval [CI], 73%-92%), and against moderate-to-severe varicella disease it was 100% (95% CI, 96%-100%). Vaccinated children with asthma or other reactive airway diseases were 7.1 times more likely to have varicella than were vaccinated children without reactive airway diseases (95% CI, 2.4-21.3). CONCLUSIONS: Varicella vaccine administered under routine conditions in physicians' offices was highly effective in preventing varicella in an outbreak characterized by intense exposure. The role of asthma and other reactive airway diseases as risk factors for varicella disease and vaccine failure deserves to be investigated further.

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