Immunogenicity and Safety Study of the 3rd Booster Dose Using the High or Medium Dose of Inactivated (CoronaVac®) Vaccine in Healthy Adults in Turkey

Inclusion Criteria: * Healthy adults aged 18 years and above, who have received 2 prior doses of inactivated COVID-19 vaccine (CoronaVac®) or mRNA COVID-19 vaccine (Comirnaty), with the second dose given 3-8 months (for CoronaVac® recipients) and 6-8 months (for Comirnaty recipients) prior to the day of booster vaccination in the present study; * Antigen rapid testing negative at the day of enrollment on site or with a PCR negative result within 72 hours before enrollment; * 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; * The participants are planning to be living in Turkey for the following 1 year after attending this study; * 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 currently with the 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 3 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 14 days prior to vaccination in this study; * Receipt of inactivated or subunit vaccines in the past 7 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 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.