A randomized, double-blind, controlled phase 3 trial was conducted during a COVID-19 outbreak after the initial, stringent zero-Covid policy was relaxed in three provinces. Eligible adults aged ≥18 years who had received three doses of inactivated COVID-19 vaccines 6 months earlier were randomly assigned in a 1:1 ratio to receive either one intramuscular injection of RQ3013 or ZF2002 vaccine. The primary end point was protection against PCR-confirmed symptomatic SARS-CoV-2 infection with onset at least 7 days after the booster. A total of 3,167 and 3,169 eligible participants received one dose of RQ3013 or ZF2002 vaccine, respectively. COVID-19 illness was confirmed in 91 participants in the ZF2002 group (11.8 per 100 person years
95% confidence interval [CI]: 9.6-14.6) and in 45 participants in the RQ3013 group (5.7 per 100 person-years
95% CI: 4.3-7.7) during a 4-month follow-up, resulting in a relative efficacy of 51.7% (95% CI, 30.9-66.2%) (