In Turkey, a single-dose varicella vaccine was introduced into the National Immunization Program in 2013. Before this implementation, varicella vaccine had been available in the private sector since 2000. However, varicella outbreaks continued to occur in preschools and elementary schools. We investigated a varicella outbreak to estimate the effectiveness of 1-dose varicella vaccine and to evaluate potential risk factors for breakthrough disease. This study was carried out during a varicella outbreak in 3 preschools in İzmir, Turkey, in April 2016. Using questionnaires, data including children`s medical and vaccination histories were collected from their parents. Attack rates in vaccinated and unvaccinated children were calculated and the analyses of vaccine effectiveness and of risk factors for breakthrough disease were conducted. A total of 124 children were enrolled in the study. Of the 124 children, 77 (62%) had received 1-dose varicella vaccine before the outbreak. Varicella developed in 34 of 124 children during the outbreak, and 18 of them (53%) had breakthrough varicella. The attack rate was 23.4% among vaccinated children and 34% among unvaccinated children. The effectiveness of single-dose varicella vaccine was 33.6% against varicella disease of any severity and 82.5% against moderate or severe varicella. Children vaccinated 5 or more years before the outbreak had 3.5 times the risk of disease than those who had been vaccinated more recently (OR 3.5 [95% CI, 1.08-11.5]); p= 0.046). Age at vaccination ( < 15 months vs.≥15 months) and the brands of varicella vaccine were not associated with the increased risk of breakthrough varicella. Our study suggests that one-dose of varicella vaccine is not sufficient to prevent school outbreaks. A 2-dose varicella vaccination program may help to prevent varicella outbreaks and achieve effective control of the disease.

Keywords: breakthrough varicella, outbreak, preschool children, vaccine effectiveness, varicella vaccine