ABSTRACT
Objective:To investigate the protective effect of two doses of varicella vaccine in a population at high risk for varicella.Methods:The case group involved all varicella cases in classes with no less than three patients during varicella outbreaks in Tianjin schools from 2017 to 2019. A case-control study was carried out in a 1∶3 ratio. According to the seat position of each case, three nearby students who did not suffer from varicella were selected as controls. The protective effect of two doses of varicella vaccine was evaluated.Results:The total protective effect of varicella vaccine was 46.0% (95% CI: 42.7%-49.0%). The protective effect was 44.7% (95% CI: 41.2%-48.0%) for one-dose varicella vaccine and 80.0% (95% CI: 63.1%-89.2%) for two-dose varicella vaccine. For two doses of varicella vaccine, the protective effect was 87.6% (95% CI: 52.0%-96.8%) after 0-2 years and 76.6% (95% CI: 33.6%-91.5%) after 3-5 years.Conclusions:The protective effect of two doses of varicella vaccine is better than that of one dose. However, both one-dose and two-dose vaccines show decreased protective effect over time. A booster immunization could be considered in high-risk population with longer intervals since the last vaccination.