The study was to evaluate the safety, immunogenicity and lot-to-lot consistency of live attenuated varicella vaccine in Chinese population aged 1–3 years. The double-blind, randomized phase III trial was conducted in Henan Province, China. In total, 1197 subjects were included in this study. Subjects were randomly assigned into four groups in a 2:2:2:1 ratio to receive one of the three lots of commercial scale (CS) vaccine or the licensed pilot scale (LPS) vaccine. Seroconversion rate and neutralizing antibody titers (NATb) were assessed at day 0 pre-vaccination and at day 30 post-vaccination. Safety data were recorded for 30 days post-vaccination. After vaccination, the geometric mean titers (GMTs) of the three CS groups were 25.04 (95% confidence interval [CI], 22.85 to 27.44), 24.47 (95% CI, 22.35 to 26.78) and 25.88 (95% CI, 23.61 to 28.36), respectively (P = 0.6928). The ratio of GMTs adjusted for covariates of each pair of lots were all between 0.67 to 1.50 in susceptible subjects. The difference of seroconversion rate between pooled CS group and LPS group was 3.82 (95% CI, 0.55 to 8.81). Meanwhile, the percentage of solicited local, systemic and unsolicited adverse reactions showed no difference across the four groups, and most of the adverse reactions were mild or moderate in intensity. The CS group was comparable to the LPS group in safety and immunogenicity. The consistency of three consecutive CS lots was reliable. Moreover, the CS group was non-inferior to the LPS group.

Background: In recent years, breakthrough varicella cases among individuals who have received a single dose of varicella vaccine (VarV) have increased notably, suggesting that the long-term protection following a one-dose VarV regimen requires further investigation. This study aims to evaluate the immune persistence following a single dose of Sinovac VarV at 5 and 8 years post-vaccination. Methods: In this Phase 4, open-label, observational follow-up study, participants aged 1 to 12 years (referring to the age of vaccination) who had received a single dose of either Sinovac VarV or placebo in the previous phase 3 trial were enrolled in a 1:1 ratio. Blood samples were collected 5 years and 8 years post-vaccination to measure antibody levels against varicella using the fluorescent antibody to membrane antigen (FAMA) method. A total of 487 and 422 participants were included in the 5-year and 8-year immune persistence analyses, respectively. The endpoints comprised the seropositive rates (≥1:4 and ≥1:8) and the geometric mean titers (GMTs) of varicella antibodies at both 5 and 8 years following vaccination. Results: Varicella antibody levels declined from 30 days post vaccination but remained stable from 5 to 8 years. Five years after vaccination, the seropositive rates (≥1:4) of varicella antibody were 100% in the VarV group and 80.83% in the placebo group; for ≥1:8, the rates were 89.07% and 64.17%, respectively. The corresponding GMTs were 1:13.67 and 1:7.71, respectively. Eight years after vaccination, the seropositive rates (≥1:4) were 99.54% in the VarV group and 90.69% in the placebo group; for ≥1:8, they were 88.53% and 74.51%, with GMTs of 1:13.52 and 1:9.91, respectively. Eight years post-vaccination, the seropositive rates and antibody levels in the VarV group remained nearly the same as by 5 years. Conclusions: Sinovac VarV can confer good immune persistence for up to 8 years following a single dose of vaccination in children aged 1 to 12 years. However, given the declining trend in antibody levels over time, revaccination may be needed to maintain protective immunity.

Aim: To evaluate the immunogenicity and safety of a booster dose of live attenuated varicella vaccine (VarV) manufactured by Sinovac (Dalian) Vaccine Technology Co. Ltd., and the immune persistence of a primary dose in 2- to 6-year-old children. Methods: A phase IV, open-label study was conducted in China. Children previously vaccinated with a single dose of VarV at 1~3 years old received one dose of homologous VarV in the first year, the second year, or the third year after the primary immunization as booster immunization. Immune persistence was evaluated in an immune persistence analysis set, while immunogenicity was evaluated in a per-protocol analysis set, and safety was evaluated in a safety analysis set. The primary endpoint was the seropositive rate and the seroconversion rate of VarV antibody. The trial was registered at ClinicalTrials.gov (NCT02981836). Results: From July 2018 to August 2020, a total of 849 vaccinated children received the booster vaccination of VarV, one booster dose for each child (301 vaccinated in the first year after primary immunization (Group 1), 276 vaccinated in the second year after primary immunization (Group 2), 272 vaccinated in the third year after primary immunization (Group 3)). The seropositive rates were 99.34%, 97.83%, and 98.16% in Groups 1–3, with GMTs of 1:22.56, 1:18.49, and 1:18.45, respectively. Thirty days after the vaccine booster dose, the seropositive rates of the three groups were all 100% and the seroconversion rates were 52.54%, 67.46%, and 66.67%, with GMTs of 1:68.49, 1:76.32 and 1:78.34, respectively. The seroconversion rates in Groups 2 and 3 were both higher than that in Group 1 (p = 0.0005 and p = 0.0008). The overall incidence of adverse reactions was 7.77%, with 7.64%, 8.33%, and 7.35% in Groups 1, 2, and 3, respectively. The main symptom among adverse reactions was fever, the incidence of which ranged from 5.07% to 6.64% in each group, and no vaccine-related serious adverse events occurred. Conclusions: VarV had good immune persistence in 1~3 years after primary immunization. A vaccine booster dose for children aged 1~3 years after primary immunization recalled specific immune response to varicella-zoster virus, with no safety concerns increased.

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Background: Varicella remains a common vaccine-preventable disease in China. Although Anhui Province recommended a two-dose varicella vaccine (VarV) schedule in 2021, real-world evidence on the incremental benefit of the second dose is limited. Methods: A prospective cohort study among children aged 1–12 years was conducted in Anhui Province from July 2022 to August 2025. Children aged 1–3 years who had received one dose of the human diploid cell line-based (SV-1) VarV and children aged 4–12 years whose second dose was the SV-1 VarV were enrolled in the exposed group and were compared with children who had no history of VarV and those who had received only one dose of the VarV, respectively. Varicella cases were collected through active follow-up and surveillance systems. Vaccine effectiveness (VE) and incremental VE were estimated as [1 − relative risk (RR)] × 100%, where the RRs were calculated based on the incidence densities of breakthrough varicella. Results: Overall, 50,054 participants were finally enrolled, contributing 125,351.5 person-years and 105 valid cases. The VE in children aged 1–3 years was 79.1% (95%CI: 42.8–92.4%). Among children aged 4–12 years, the incremental VE was 65.0% (95%CI: 41.9–78.9%), with incremental VEs of 60.1% (95%CI: 22.3–79.5%) for ages 4–6 years and 72.7% (95%CI: 37.8–88.0%) for ages 7–12 years. Conclusions: One-dose SV-1 VarV provided substantial protection in young children, and a second dose conferred significant additional protection in children aged 4–12 years, supporting strengthened implementation of the two-dose strategy and catch-up vaccination among school-aged children.