Varicella vaccine effectiveness during an outbreak in a partially vaccinated population in Spain

Background: An outbreak of varicella occurred between December 2004 and April 2005 in the primary school and day-care center of a town of 5430 inhabitants in Spain. Although the varicella vaccine is not included in the infant vaccination program, some children had been vaccinated before the outbreak. Study Aim: The aim of this study was to estimate varicella vaccine effectiveness during an outbreak that took place in a partially vaccinated population of children. Methods: A cohort study was carried out. Cases were identified through notification by doctors and active search. Information was gathered on the current disease, history of varicella and previous vaccinations together with age, course, school year and other sociodemographic factors. The relative risk (RR) of varicella and the contribution of the previously mentioned factors to the probability of contracting the disease was estimated for vaccinated and unvaccinated children. Results: Participation reached 96.5% (387 of 401 children) in the school and 91.2% (31 of 34 children) in the day-care center. Of 269 children with no history of varicella and with a documented vaccination record, 96 (35.7%) had been previously vaccinated. During the outbreak, 148 cases of varicella were observed. The overall attack rate was 54.4%, 22.9% in vaccinated and 72.8% in unvaccinated children. RR of varicella in vaccinated children was 0.31 (95% confidence interval [CI] = 0.21-0.46). The overall adjusted vaccine effectiveness against varicella was 69.5% (95% CI = 50.5-81.5%) and 96.9% (95% CI = 77.5-99.6%) against mild and severe forms. Only time since vaccination was associated with vaccine failure. Conclusions: In the varicella outbreak studied, we conclude that vaccine was effective in the prevention of the disease, particularly in its moderate and severe forms, but because the proportion of vaccinated children was low, an outbreak still occurred.