Background China has been using inactivated COVID-19 vaccines as primary series and booster doses to protect the population from severe to fatal COVID-19. We evaluated primary and booster vaccine effectiveness (VE) against Omicron BA.2 infection outcomes. Methods This was a 13-province retrospective cohort study of quarantined close contacts of BA.2-infected individuals. Outcomes were BA.2 infection, COVID-19 pneumonia or worse, and severe/critical COVID-19. Absolute VE was estimated by comparison with an unvaccinated group. Results There were 289,427 close-contacts ≥3 years old exposed to Omicron BA.2 cases; 31,831 turned nucleic-acid amplification test (NAAT)-positive during quarantine, 97.2% with mild or asymptomatic infection, 2.6% had COVID-19 pneumonia, and 0.15% had severe/critical COVID-19. None died. Adjusted VE against any infection was 17% for primary series and 22% when boosted. Primary series aVE in adults >18 years was 66% against pneumonia or worse infection and 91% against severe/critical COVID-19. Booster dose aVE was 74% against pneumonia or worse, and 93% against severe/critical COVID-19. Conclusions Inactivated COVID-19 vaccines provided modest protection from infection, very good protection against pneumonia, and excellent protection against severe/critical COVID-19. Booster doses are necessary to provide strongest protection.

Using a prospective, observational cohort study during the post-dynamic-COVID-zero wave in China, we estimated short-term relative effectiveness against Omicron BA.5 infection of inhaled aerosolized adenovirus type 5-vectored ancestral strain COVID-19 vaccine as a second booster dose ∼1 year after homologous boosted primary series of inactivated COVID-19 vaccine compared with no second booster. Participants reported nucleic acid or antigen test results weekly until they tested positive or completed pre-designated follow-up. After excluding subjects infected less than 14 days after study entry, relative effectiveness among the 6,576 subjects was 61% in 18-59-year-olds, 38% in ≥60-year-olds, and was sustained for 12 weeks.