Immunogenicity and Safety Study of the 3rd Booster Dose Using the High or Medium Dose of Inactivated Vaccine in Healthy Adults in in Hong Kong

Inclusion Criteria: * Healthy adults aged 18 years and above, who have received two prior doses of mRNA COVID-19 vaccine (Comirnaty®, made by BioNTech/Pfizer) as the primary vaccination, with the second dose given at least 180 days prior to the day of booster vaccination in the present study; * Evidence of a deep throat saliva (DTS) PCR negative for SARS-COV-2 within 24 hours before enrollment; * Female participants who have a negative pregnancy test on the day of vaccination, and not currently breastfeeding; * The participants are able to understand and sign the informed consent voluntarily; * The participants are willing and able to comply with all schedule visits, sample collection,vaccine plan, and other study procedures; * The participants 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 or having evidence showing a current 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; * Known allergy to vaccines or vaccine 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 three months before vaccination in this study; * Participation in other studies involving study intervention within 30 days prior to vaccination in this study; * Receipt of attenuated live vaccines in the past fourteen days prior to vaccination in this study; * Receipt of inactivated or subunit vaccines in the past seven days prior to vaccination in this study; * Acute exacerbation or presentation of stable chronic diseases (including but not limited to asthma,migraine, gastrointestinal disorder, etc.); * Acute febrile illness with oral temperature \>37.5C on the day of 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.