A PhaseⅡ Study to Evaluate the Safety & Immunogenicity of SARS-CoV-2 Alpha/Beta/Delta/Omicron Variants COVID-19 Vaccine

Firstly, 250 participants will be randomly enrolled in group A, among which the first 15 participants will be observed for 7 days after receiving the study vaccine and assessed for safety by the DSMB. If the 15 participants do not meet the study suspension/termination criteria, 15 sentinel participants in group B will be enrolled. The sentinel participants in Group B will be observed for 7 days after receiving the study vaccine, and safety is assessed by the DSMB. If the sentinel participants in group B do not meet the study suspension/termination criteria, non-sentinel participants in Group B and 15 sentinel participants in Group C aged 6-11 years old will be enrolled at the same time. The 15 sentinel participants aged 6-11 years in group C will be observed for 7 days after receiving the study vaccine and assessed for safety by the DSMB. If the study suspension/termination criteria are not met, non-sentinel participants aged 6-11 years old in group C and 15 sentinel participants aged 3-5 years old in group C will be enrolled, and sentinel participants aged 3-5 years old in group C will be observed for 7 days after receiving the study vaccine. Safety will be assessed by the DSMB and non-sentinel participants aged 3-5 years old in Group C will be enrolled if study suspension/termination criteria are not met in sentinel participants aged 3-5 years old. Participants in each group (including the first 15 participants in Group A and sentinel participants in groups B and C) will be randomly inoculated with either SCTV01E or placebo (normal saline) at a ratio of 4:1, i.e. 200 participants in each group will receive SCTV01E and 50 participants will receive placebo (normal saline). Randomized stratification factors for group A are age (18-59 years vs. ≥60 years), past COVID-19 vaccination status (completion of baseline immunization vs. Completion of booster immunization), the last type of COVID-19 vaccine received (inactivated vs. other vaccines), and the duration of prior Informed consent form (ICF) signing (6-12 months vs. 12-24 months); Randomized stratification factor for group B are past COVID-19 vaccination (completion of baseline immunization vs. Completion of booster immunization), type of COVID-19 vaccine last received (inactivated vs. other vaccines) and time since ICF last received (6-12 months vs. 12-24 months); Random stratification factors for group C are age (3-5 years vs. 6-11 years), past COVID-19 vaccination (completion of baseline immunization vs. Completion of enhanced immunization), the last type of COVID-19 vaccine received (inactivated vs. other vaccines), and the duration of prior Informed consent form (ICF) signing (6-12 months vs. 12-24 months).