Explore the optimal timing for administering the second dose of the varicella vaccine in China.

The prevalence of varicella in China has been increasing annually, with a relatively high incidence rate of breakthrough cases. Administering two doses of the varicella vaccine (Varv) proves to be the most effective measure. The objective of this study is to assess the immunogenicity of two doses of the Varv at varying intervals and explore the optimal timing for administering the second dose of the Varv. Utilizing a prospective cohort study design, the quantification of varicella immunoglobulin G (IgG) antibodies' geometric mean concentrations (GMC) is conducted through glycoprotein-based enzyme-linked immunosorbent assay (gpELISA). A total of 903 infants were included in the per-protocol population. After completing the first dose of the Varv, the GMC of antibody after 1 month (Group A) was 463.8 (447.6-480.1) mIU/mL. There was a statistically significant difference in GMC and seroconversion rates among the groups (B/C/D) that received the second dose of the Varv (p < 0.05). Multiple comparisons revealed that the group with a 3-year interval between the two vaccine doses had a higher GMC of 665.2 (622.6-707.8) mIU/mL compared to the group with a 1-year interval of 611.1 (577.1-645.3) mIU/mL and the group with a 5-year interval of 564.7 (540.1-589.4) mIU/mL. To effectively prevent and control the varicella epidemic in Jiangsu Province, two dose Varv vaccination is recommended, the optimal time point for the second dose Varv is 3 years after the first vaccination.