Safety and Immunogenicity Study of Booster Vaccination With Medium-dosage or High-dosage SARS-CoV-2 Inactivated Vaccine for Prevention of COVID-19

Inclusion Criteria: * Adults aged 18-59; * Proven legal identity; * The subjects can understand and voluntarily sign the informed consent form and be willing to complete the study in accordance with the study plan; * Have received two doses of inactivated SARS-CoV-2 vaccine(CoronaVac) manufactured by Sinovac Research \& Development Co., Ltd at an interval of 21 to 35 days and is currently 6-8 months (180 to 240 days) after the second dose. Exclusion Criteria: * History of SARS-CoV-2 infection(laboratory confirmed); * Have received three and more doses of inactivated SARS-CoV-2 vaccine; * Severe adverse reactions, such as urticaria, dyspnea, and angioneurotic edema , occurred during the primary immunization; * Autoimmune disease such as systemic lupus erythematosus or immunodeficiency / immunosuppression such as AIDS, post-transplant; * Severe chronic diseases, severe cardiovascular diseases, hypertension and diabetes that cannot be controlled by drugs, liver or kidney diseases, malignant tumors, etc; * Severe neurological disease (epilepsy, convulsions or convulsions) or mental illness; * Diagnosed abnormal blood coagulation function (eg, lack of blood coagulation factors, blood coagulopathy, abnormal platelets) or obvious bruising or blood coagulation; * Immunosuppressive therapy, cytotoxic therapy, inhaled corticosteroids(excluding allergic rhinitis corticosteroid spray therapy, acute noncomplicated dermatitis superficial corticosteroid therapy) in the past 6 months; * History of alcohol or drug abuse; * Receipt of blood products within in the past 3 months; * Receipt of other investigational drugs in the past 30 days; * Receipt of attenuated live vaccines in the past 14 days; * Acute diseases or acute exacerbation of chronic diseases within 7 days prior to booster vaccination; * Axillary temperature \>37.0°C; * Already pregnant or are breastfeeding, planning to get pregnant within 1months (according to subjects' self-report and urine pregnancy test results); * Participated in other clinical trials prior to enrollment and during the follow-up period, or planned to participate in other clinical trials during the clinical trial period; * According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.