Vaccine Effectiveness and Severity of Varicella Among Previously Vaccinated Children During Outbreaks in Day-Care Centers With Low Vaccination Coverage

Abstract

Varicella vaccine effectiveness (VE) during outbreaks has been reported to be 71-100% against any disease and >90% against moderate/severe disease even in day-care centers (DCCs) and schools with low vaccination rates. A recent report suggested an effectiveness rate of 44% during a DCC outbreak despite a high vaccination rate. To reassess vaccination coverage, VE and severity of disease among previously vaccinated children after exposure during DCC outbreaks in northern Israel, where vaccination rates are low. During January to June 2003, active surveillance for varicella among children in northern Israel revealed outbreaks in 8 DCCs with children 3-6 years of age. Data concerning symptoms of the disease and the age at vaccination (for previously vaccinated children) were obtained from parents and health care providers for children who contracted the disease. Analysis of VE was limited to children who were continuously enrolled in DCCs during the outbreaks. The overall vaccination rate was 37%. The incidences of natural varicella and breakthrough varicella (BV) were 79 of 153 [52%; 95% confidence interval (CI) 44-60%] and 37 of 89 (41.5%; 95% CI 31-52%), respectively. VE was 20% (95% CI 0-40%) against disease of any severity and 93.4% (95% CI 75-98%) against moderate/severe disease. Ninety-four percent and 14% of children with BV and natural varicella, respectively, had mild disease (P < 0.001). The odds ratio for BV was 17 (95% CI 2.18-118) for children vaccinated >2 years before the outbreak. During varicella outbreaks in DCCs with low vaccine coverage, previous vaccination provided poor protection against chickenpox, mostly among children who had been vaccinated >2 years earlier, but the disease appeared to be much milder among children with BV than among nonvaccinated children.