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.