Postlicensure study of varicella vaccine effectiveness in a day-care setting

Background. Varicella vaccine has been licensed for use in the United States since the spring of 1995. The acceptance of the vaccine and its effect on varicella incidence in children is important, Aim. To document the effectiveness of the varicella vaccine in children attending day care in 11 centers in North Carolina. Methods. a dynamic cohort study design was used in 11 day-care centers in North Carolina, Multiple cross-sectional evaluations were performed and children were noted to be vaccinated or not and diseased or not. Vaccine effectiveness was estimated by comparing the varicella attack rate in the vaccinated with the varicella attack rate in the unvaccinated, Person time was used as the denominator for all calculations. Results. During the study period February 1, 1996, to September 1, 1997, 134 cases of varicella occurred in the unvaccinated and 11 cases occurred in the vaccinated children. The attack rates in the vaccinated and unvaccinated were 2.49 and 14.66, respectively, for an overall vaccine effectiveness of 83% for mild/moderate disease. Conclusions. In the day-care setting varicella vaccine demonstrated benefit in preventing and modifying wild-type varicella disease.