POSTLICENSURE EFFECTIVENESS OF VARICELLA VACCINE DURING AN OUTBREAK IN A CHILD-CARE CENTER

Context.-Because lyophilized varicella vaccine must be stored frozen a t -15 degrees C or less (less than or equal to 5 degrees F) and admini stered within 30 minutes after reconstitution, the potential exists fo r decreased vaccine effectiveness when the vaccine is used under field conditions. Objectives.-To describe an outbreak of varicella in a chi ld care center and to determine postlicensure effectiveness of varicel la vaccine. Design.-Retrospective cohort study. Setting.-A child care center in DeKalb County, Georgia, in 1996. Participants.-Of the 184 ch ildren 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 va ccine effectiveness was calculated among children aged 12 months or ol der (eligible for vaccination) using the cohort method. Results.-The o utbreak started on January 17, 1996, and lasted 15 weeks. Of the 148 e ligible children, 81 (55%) developed varicella, Cases among children y ounger than 12 months (n=7) were more severe than cases among older ch ildren, Varicella occurred in 9 (14%) of 66 vaccinated children and 72 (88%) of 82 unvaccinated children. Varicella was less severe and resu lted in fewer days of absence from the child care center among vaccina ted 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 a irway 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 condition s in physicians' offices was highly effective in preventing varicella in an outbreak characterized by intense exposure. The role of asthma a nd other reactive airway diseases as risk factors for varicella diseas e and vaccine failure deserves to be investigated further.