Objective  To identify the risk factors for a varicella outbreak in a kindergarten in Nanjing, Jiangsu province, and evaluate the efficiency of varicella attenuated live vaccine (VarV) in the children.

  Methods  Case definition was established and case finding was conducted. The demographic data, medical records, epidemiological history, immunization history of varicella and other data were collected from the cases. The distribution characteristics of cases and the differences in attack rate were explored. The immunization history of varicella and history of varicella were collected from children in classes with a high attack rate of varicella. A retrospective cohort study was used to analyze the differences in attack rate of varicella between vaccinated children and unvaccinated children, and the efficiency of VarV as well as the association between the time interval of the vaccination and efficiency of VarV were evaluated.

  Results  A total of 61 cases of varicella were diagnosed. The attack rate of varicella was significantly higher in children (10.87%, 59/543) than in kindergarten staff (2.94%, 2/68) (χ²=4.22, P=0.040). The index case was a head teacher, who continued to work after the illness onset without reporting and was not detected by the routine health surveillance of kindergarten. The school perform no intervention measures after this teacher's sick leave. This outbreak lasted for two months, resulting in 5 generations of the cases with a median generation interval of 15 days. The attack rates among different classes showed significant difference (χ²=256.98, P<0.001). Among 61 cases, 8 (13.11%) cases were breakthrough cases, who had received one-dose VarV. For these 8 breakthrough cases, the median time interval between vaccination and illness onset was 4.11 years. The results showed that VarV efficiency in this outbreak was 65% (95% CI: 24%–84%), and the vaccine efficiency decreased in the cases with the interval of more than 3 years between the first dose vaccination and this outbreak or illness onset.

  Conclusion  This was a varicella outbreak in kindergarten with index case being a school staff. The potential risk factors for this outbreak included failure to identify and isolate case timely, inappropriate distribution of classrooms, low vaccination coverage, and long time after one-dose vaccination.