Inclusion Criteria:  Healthy adults aged 18 years and above, who have received 2 prior doses of CoronaVac® with an interval between those doses of 21-56 days, with the second dose given 5-8 months prior to the day of vaccination in the present study; The participants are able to understand and sign the informed consent voluntarily; Female participants who have a negative pregnancy test on the day of vaccination, and not currently breastfeeding; Must be willing to provide verifiable identification (in accordance with the local regulations), has means to be contacted and to contact the investigator during the study.  Exclusion Criteria:  History of confirmed infection of SARS-CoV-2 prior to randomization; Any prior administration of another investigational coronavirus vaccine or current/planned simultaneous participation in another interventional study to prevent or treat COVID-19; Allergy to vaccines or vaccine/placebo ingredients, and serious adverse reactions to vaccines, such as urticaria, dyspnea, angioneurotic edema; Serious chronic disease, serious cardiovascular disease, hypertension and diabetes that cannot be controlled by drugs, hepatorenal disease, malignant tumor, etc; Acute central nervous system diseases such as encephalitis/myelitis, acute disseminating encephalomyelitis, and related disorders; Receipt of blood/plasma products or immunoglobulins in the past 3 months before vaccination; Participation in other studies involving study intervention within 30 days prior to vaccination; Receipt of attenuated live vaccines in the past 14 days prior to vaccination; Receipt of inactivated or subunit vaccines in the past 7 days prior to vaccination; Acute exacerbation or presentation of stable chronic diseases (including but not limited to asthma, migraine, gastrointestinal disorder, etc); Acute febrile illness with axillary temperature >37.5°C on the day of vaccination; enrollment could be considered if the fever is absent for 72 hours prior to vaccination; According to the investigator's judgment, the participant has any other factors that might interfere with the results of the clinical trial or pose additional risk to the participant due to participation in the study