First-generation monovalent vaccines blocked transmission of Delta and Omicron variants of SARS-CoV-2 in China

While effectiveness of monovalent inactivated SARS-CoV-2 vaccines on preventing infection and severe disease has been well-characterized, their effectiveness on reducing infectivity of infected individuals and thereby blocking onward transmission to close contacts remains poorly understood. Leveraging contact-tracing data obtained during the Delta outbreaks in Guangzhou (May–June 2021) and Yangzhou (July–August 2021), as well as the Omicron outbreak in Guangzhou (April 2022), we used individual-based Bayesian transmission models to estimate vaccine effectiveness on reducing susceptibility (VES) and infectivity (VEI) across various contact settings, adjusting for tertiary transmission, case importation, and exposure history. Results: Based on 323 primary cases and 53,895 close contacts, we find no evidence for VES (comparing 0, 1, and ≥2 doses) but significant VEI (comparing ≤1 vs. ≥2 doses) against both Delta and Omicron variants. Compared to ≤1 dose of inactivated monovalent vaccines, ≥2 doses are associated with 45% (95% Credible Interval [CrI]: 15%, 66%) lower infectivity for the Delta variant and 54% (95% CrI: 34%–80%) for the Omicron variant respectively. Household and entertainment are associated with the highest baseline transmission risks. The most susceptible age group is older adults (60+ years) during the Delta outbreaks but children (0–19 years) during the Omicron outbreak. First-generation monovalent inactivated vaccines offer sustainable community-level protection against novel variants by reducing infectivity of infected individuals, representing a critical but understudied component of the herd immunity. Inactivated COVID-19 vaccines are known to prevent severe disease, but their ability to reduce how contagious infected people are remains poorly understood. Using detailed contact-tracing data from Delta and Omicron outbreaks in Guangzhou and Yangzhou, China (2021-2022), we used statistical models to estimate vaccine effects on both susceptibility to infection and infectivity of infected individuals. We found that while inactivated vaccines provided minimal protection against getting infected, they substantially reduced onward transmission by 45% for Delta and 54% for Omicron among individuals with two or more doses. Households and entertainment showed the highest transmission risk. These findings indicate that first-generation inactivated vaccines can suppress community transmission by reducing how infectious vaccinated cases are. Tsang, Jing, et al. use individual-based Bayesian transmission models with contact-tracing data from Delta and Omicron outbreaks in China. They find that first-generation monovalent inactivated vaccines provide minimal protection against infection but reduce onward transmission by 45-54%.