Association of the publicly funded two-dose varicella vaccine policy with varicella incidence in Shanghai: an interrupted time series study

Background Two-dose varicella vaccine (VarV) schedule has not been included in the China’s National Immunization Program. Shanghai was the first provincial-level region to implement a publicly funded two-dose VarV policy in 2018. The study assessed the policy’s impact on varicella incidence in Shanghai.Methods Varicella surveillance data from two comparable districts in Shanghai were analyzed using an interrupted time series design. Counterfactual and age-stratified analyses were further conducted.Results A total of 26,241 varicella cases were reported during 2012–2024. The overall age-standardized varicella incidence rate in Shanghai decreased from 104.8 to 74.0 cases per 100,000 person-years (RR = 0.75, p < 0.001). In 2023–2024, varicella incidence exhibited a rebound trend (p = 0.024). The counterfactual model estimated a 52.1% reduction. The age-standardized monthly incidence rate showed a non-significant immediate decrease of 11.09 per 100,000 individuals (p = 0.053), followed by an additional monthly decrease of 0.75 cases per 100,000 individuals (p < 0.001). Age-stratified analysis indicated an immediate reduction in varicella incidence among children aged 0–4 and 5–9 years, and a significant long-term decline across all age groups (each p < 0.001).Conclusion Following policy implementation in 2018, varicella incidence in Shanghai declined substantially and showed a sustained downward trend. The findings support optimization of VarV strategies in China.