SINOVAC Varicella Booster Dose Study Provides New Evidence on Timing for Second Dose Administration

BEIJING, China, July 31, 2026 — A study evaluating the optimal timing of a booster dose of varicella vaccine among school-aged children has been published in the Chinese Journal of Epidemiology. The research was jointly conducted by the Jiangsu Provincial Center for Disease Control and Prevention, Xuzhou Center for Disease Control and Prevention, the School of Public Health of Nanjing Medical University, and the Chinese Center for Disease Control and Prevention.

The study systematically evaluated the immunogenicity and safety of SINOVAC's varicella vaccine, live when administered as a booster dose at different intervals following primary immunization, providing new evidence to support optimization of the two-dose varicella vaccination strategy and selection of an appropriate timing for booster dose administration.

Study Focuses on Administration Timing of Booster Dose in School-Aged Children

Varicella, caused by varicella-zoster virus, is a highly contagious disease that primarily affects children and can easily lead to outbreaks in childcare facilities and schools. Although most cases are mild, some children may develop complications such as secondary bacterial infections, pneumonia, or encephalitis.

Previous studies have shown that one dose of varicella vaccine effectively reduces the risk of varicella and moderate-to-severe disease. However, antibody levels may gradually decline over time, and breakthrough infections may still occur.

For children aged 7–12 years, increasing social contact and participation in group activities make identification of an appropriate booster dose interval important for reducing the risk of varicella transmission during the school years.

412 Children Enrolled Across Four Booster Dose Interval Groups

The open-label study was conducted in Xuzhou, Jiangsu Province, between October 2024 and October 2025. It enrolled children aged 7–12 years who had completed primary immunization with one dose of varicella vaccine.

Participants were grouped according to the interval between their first dose and the study-administered second dose:

• Approximately 3 months;

• 18–30 months;

• 36–48 months;

• 60–72 months.

A total of 412 children were enrolled. Blood samples were collected before booster vaccination and 30 days afterward. Immunogenicity was evaluated using varicella antibody geometric mean titers (GMTs), geometric mean increases (GMIs), seropositivity rates, and seroconversion rates. Safety was assessed by actively monitoring adverse reactions within 30 days after vaccination.

The primary immunization dose was administered using varicella vaccines from multiple domestic manufacturers, while all booster doses were SINOVAC's varicella vaccine, live.

Booster Dose Induced Robust Immunogenicity

Before booster vaccination, seropositivity rates remained consistently high across all four groups, ranging from 92.05% to 94.87%. Meanwhile, GMTs declined progressively with increasing time since primary immunization:

• 3-month group: 27.03;

• 18-month group: 21.59;

• 36-month group: 18.29;

• 60-month group: 14.33.

These findings indicate that although most children maintained detectable antibodies after primary immunization, antibody levels gradually declined over time, supporting the need for booster vaccination.

Thirty days after receiving SINOVAC's varicella vaccine, live as a booster dose, GMTs increased significantly in all four groups:

• 3-month group: 87.35;

• 18-month group: 113.55;

• 36-month group: 141.80;

• 60-month group: 129.01.

Compared with pre-booster levels, antibody titers increased 3.23-fold, 5.26-fold, 7.75-fold, and 9.00-fold, respectively (P<0.001), demonstrating strong booster immune responses regardless of the vaccination interval.

Following the booster dose, seropositivity reached 100% in all groups, while seroconversion rates were 60.26%, 71.60%, 77.27%, and 65.91%, respectively, with no statistically significant differences among groups (P>0.05).

Among the four groups, the 36–48-month interval group achieved the highest post-booster GMT. The authors concluded that administering a booster dose 36–48 months after primary immunization may provide a more robust antibody response while helping maintain protection before children enter high-exposure school environments.

Favorable Safety Profile

Among the 412 children included in the safety analysis, the incidence of vaccine-related adverse reactions was 0.73%, and all events were Grade 1 or Grade 2, consisting primarily of local injection-site reactions or mild systemic reactions.

No Grade 3 or higher adverse reactions or serious adverse events were observed, and no significant differences in adverse reaction rates were found among the four booster interval groups.

These findings demonstrate that SINOVAC's varicella vaccine, live exhibited favorable immunogenicity and safety when administered as a booster dose in children aged 7–12 years.

Supporting Optimization of the Two-Dose Varicella Vaccination Strategy

The study showed that administering a second dose between 3 months and 60–72 months after primary immunization elicited strong immune responses across all interval groups.

The results further suggest that completing the second dose before children enter primary school or other high-exposure group settings may help reduce periods of lower immunity and improve protection among school-aged children.

Overall, the findings provide scientific evidence to support implementation of a two-dose varicella vaccination strategy in China and further evaluation of the optimal timing for booster dose administration.

SINOVAC will continue collaborating with disease prevention and control institutions, research organizations, and healthcare providers to conduct studies on vaccine immunogenicity, effectiveness, safety, and immunization strategies, generating high-quality evidence to support disease prevention and public health decision-making.

About the Publication

Sun X, Liu D, Zhang L, et al. Immunogenicity and Safety of Varicella Attenuated Live Vaccine with Booster Immunization at Different Intervals in Children Aged 7–12 Years. Chinese Journal of Epidemiology. 2026;47(6):1128–1134.

Disclaimer

This article summarizes findings from a published scientific study and is intended for informational purposes only. It does not replace professional medical advice or individual vaccination recommendations. Vaccination should be administered in accordance with local immunization policies, approved product labeling, and guidance from qualified healthcare professionals.

About SINOVAC

Sinovac Biotech Ltd. (SINOVAC) is a global biopharmaceutical company headquartered in China. Guided by its mission to “Supply Vaccines to Eliminate Human Diseases,” the Company is dedicated to the research, development, manufacturing and supply of vaccines and related biological products, providing vaccine products and related solutions for disease prevention and control worldwide.

SINOVAC has a diversified vaccine portfolio across multiple disease areas, including influenza, viral hepatitis, varicella, hand, foot and mouth disease (HFMD), poliomyelitis, rabies, tetanus, and pneumococcal disease. Three of SINOVAC's vaccines have been prequalified by the World Health Organization: the inactivated hepatitis A vaccine Healive®, the Sabin-strain inactivated poliomyelitis  vaccine (sIPV), and the live attenuated varicella vaccine.

SINOVAC has extensive experience in vaccine research, development and commercialization for emerging infectious diseases and public health emergencies. The Company initiated vaccine R&D in response to major public health threats, including SARS, H5N1, H1N1, and COVID-19, and developed the world's first inactivated SARS vaccine (with Phase I clinical trials completed), China's first inactivated pandemic influenza vaccine against H5N1 (Panflu®), the world's first H1N1 influenza vaccine (Panflu.1®), and CoronaVac®, the world's most widely used inactivated COVID-19 vaccine. Beyond its marketed portfolio, SINOVAC continues to advance its R&D pipeline, spanning combination vaccines, recombinant protein vaccines, mRNA technologies, antibody-based products, and other next-generation technologies and product platforms, while expanding its capabilities in disease prevention and biopharmaceuticals.

With a long-standing commitment to innovation and global health, SINOVAC continues to deepen collaboration with research institutions, international organizations, and local partners while expanding its global presence. Through technological collaboration, localized production, and industrial partnerships, the Company aims to improve access to high-quality vaccines across regions, better address local disease prevention and healthcare needs, and strengthen preparedness for future public health challenges.

For more information, please see the Company's website at www.sinovac.com.