The study is a multicenter, randomized, observer-blinded, active-controlled trial. About 300 eligible participants will be randomized equally (1:1:1) into 20 μg ReCOV group, 40 μg ReCOV group, and the control vaccine [COVID-19 Vaccine (Vero Cell), Inactivated] group, respectively, stratified by age (18 to < 50 years, ≥ 50 to ≤ 65 years) and the duration since the last dose of primary vaccination (90~180 days, 181~365 days).~All Participants will be observed for 30 minutes after study vaccination at study site if Participants will be asked to record solicited AEs within 7 days and unsolicited AEs within28 days. After 28 days post the study vaccination，all participants will have 2 on-site safety follow-up Visits on Day 90 (±15days) and on Day 180 (±15days) after the study vaccination to collect serious adverse evens (SAEs), adverse events of special interest (AESIs). During the study period, they can spontaneously report any adverse events (AEs) they experienced at any time.
This is a multicenter, randomized, observer-blinded, active-controlled Phase II study to evaluate the immunogenicity, safety and reactogenicity of the low dose (20μg) and high dose (40μg) of ReCOV, when administered as a booster to healthy adults aged 18 to 65 years who have completed two or three doses vaccination (prioritized two doses). Estimate 300 participants.