Analysis on the Epidemiological Characteristics of Breakthrough Varicella Cases and Incremental Effectiveness of 2-Dose Varicella Vaccine in China

Background: A 2-dose varicella vaccination strategy has been implemented in Shanghai, China since 2018. This study aims to analyze the epidemiological characteristics of breakthrough varicella cases and to evaluate the incremental effectiveness of the 2-dose varicella vaccination among Chinese children. Methods: A retrospective investigation was conducted among native children born between 2015 and 2019 in the Pudong New area of Shanghai, China. From 2016 to 2023, demographic information and data regarding varicella vaccination were collected through the Shanghai Immunization Information System, while information on varicella infections was obtained from the China Information System for Disease Control and Prevention. The incremental vaccine effectiveness (VE) for varicella was defined as (1 − relative risk (RR)) × 100%, where RRs were calculated based on the rate of varicella infections. Results: A total of 519,951 local children were enrolled in the cohort analysis. The overall rate of breakthrough varicella infections was found to be 0.25%, corresponding to 1182 cases. Specifically, the rates of breakthrough varicella infections for individuals who received 1-dose and 2-dose VarV were 0.39% (977 cases) and 0.10% (205 cases), respectively. The average ages of onset for these infections were 2.13 ± 0.99 years and 5.52 ± 1.18 years, respectively. Furthermore, the breakthrough varicella infection rate among individuals born between 2015 and 2019 exhibited a decline, decreasing from 0.52% to 0.26% for those who received one dose of VarV, and from 0.23% to 0.01% for those who received two doses. The VE against all varicella was 85% (95% confidence interval: 84–86) for one dose and 96% (95% confidence interval: 96–97) for two doses, and the incremental VE was 75% (95% confidence interval: 71–79) compared with 1-dose. Conclusions: the 2-dose VarV vaccination strategy provided excellent protection to prevent varicella, and the universal 2-dose regimen of the varicella vaccine should be recommended to prevent varicella disease among children in China.