The Impact and Effectiveness of a Two-Dose Varicella Immunization Program Among School Children: Evidence from Quasi-experimental Analyses

Abstract Introduction Varicella, caused by the varicella zoster virus, poses a significant public health challenge, particularly among children. To address this, the city of Shenzhen, China, introduced a publicly funded vaccination program offering free two-dose varicella vaccines to eligible school-aged cohorts. This study aims to evaluate the program’s impact on disease incidence, vaccination uptake, and the comparative effectiveness of two-dose versus single-dose regimens. Methods We used the individual-level data from official immunization and surveillance systems, covering pupils born between 1 September 2014 and 31 August 2015. Applying a difference-in-differences approach, we compared the disease incidence and vaccination uptake between the eligible cohort and ineligible cohort. Additionally, to further evaluate the effectiveness of the two-dose vaccination schedule, we implemented a regression discontinuity design using the policy cutoff date. Result The policy reduced varicella incidence by 22.96% (95% CI 8.98–34.79%, p < 0.05) in the eligible cohort relative to the ineligible one, while boosting first- and second-dose coverage by 34.97% (95% CI 28.19–2.11%, p < 0.05) and 98.05% (95% CI 91.98–104.31%, p < 0.05), respectively. The regression discontinuity design indicated that the two-dose schedule conferred 75.19% (95% CI 70.62–79.06%) relative vaccine effectiveness against breakthrough varicella after a single dose. This study’s main limitation is that, while reporting of varicella cases is mandatory, underreporting and misdiagnosis remain possible, potentially underestimating true incidence. Conclusions These findings demonstrate the program’s success in reducing disease burden and enhancing immunization rates, underscoring the superior protection of two-dose vaccination and providing strong evidence for policymakers to expand such initiatives.