Epidemiological Characteristics of Breakthrough Varicella Cases and Vaccine Effectiveness among Children in Hechi Based on Surveillance Data from 2019 to 2024

Objective: Relying on the infectious disease and immunization program surveillance database, this study systematically analyzed the epidemiological distribution of breakthrough varicella infection cases among children under 15 years old in Hechi, Guangxi from 2019 to 2024, quantified the long-term protective efficacy of different doses of live attenuated varicella vaccine, and provided field epidemiological evidence for optimizing local varicella immunization programs and precise prevention and control of respiratory infectious diseases in schools and kindergartens. Methods: Individual records of confirmed varicella cases among children under 15 years old in Hechi from January 1, 2019 to December 31, 2024 were extracted from the National Notifiable Infectious Disease Reporting System of China. Meanwhile, complete vaccination archives of local children born between 2011 and 2020 recorded in the Guangxi Immunization Information Management System were retrieved. Record linkage between the two systems was performed using matching identifiers including ID number, date of birth, residential county and guardian contact information; duplicate records were deduplicated, and the number of cases excluded due to unverifiable vaccination records was documented. Descriptive epidemiological methods were adopted to sort out the population, temporal and regional distributions of cases. The chi-square test for trend was used to analyze the annual variation trend of breakthrough cases. Spearman’s rank correlation was applied to explore the correlation between vaccination coverage and the proportion of breakthrough cases. Cohort data were used to calculate the vaccine effectiveness (VE) of 1-dose and 2-dose vaccination schedules. 95% confidence intervals for breakthrough rates and VE estimates were calculated, and VE values were stratified and adjusted by birth cohort, age, county/district and calendar year. Results: A total of 27,417 varicella cases were reported among children under 15 years old in Hechi from 2019 to 2024, including 6,470 breakthrough varicella cases accounting for 23.60% of all cases. Among breakthrough cases, 6,154 occurred after single-dose vaccination (95.12% of all breakthrough cases), while only 316 cases occurred after complete 2-dose vaccination (4.88%). The proportion of breakthrough cases in total varicella cases showed an overall fluctuating upward trend with a slight decline in 2023 after the peak in 2022 from 2019 to 2024 (χ² for trend = 282.77, P < 0.001), peaking at 29.86% in 2024.