This is a randomized, double-blinded, phase Ⅳ clinical trial of COVID-19 vaccine (Vero cell), Inactivated manufactured by Sinovac Life Sciences Co. , Ltd. The purpose of this study is to evaluate the safety and immunogenicity of booster vaccination using high-dosage or medium-dosage of COVID-19 vaccine (Vero cell), Inactivated in populations who have completed primary immunization 5-9 months

This study a randomized, double-blinded, phase Ⅳ clinical trial of COVID-19 vaccine (Vero cell), Inactivated.The purpose of this study is to evaluate the safety and immunogenicity of booster vaccination using high-dosage or medium-dosage of COVID-19 vaccine (Vero cell), Inactivated in populations who have completed primary immunization 5-9 months.A total of 340 subjects who have completed primary immunization using two-dose Coronavac® 5-9 months were be enrolled.All of subjects were be randomly divided into two groups in a 1:1 ratio to receive high-dosage or medium-dosage of COVID-19 vaccine (Vero cell), Inactivated.

Inclusion Criteria:

• Healthy participants aged 18 years and above;
• 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 COVID-19 vaccine (CoronaVac®) manufactured by Sinovac Research & Development Co., Ltd and that is currently 5-9 months after the second dose.

Exclusion Criteria:

• History of SARS-CoV-2 infection(laboratory confirmed);
• Have received three and more doses of inactivated COVID-19 vaccine;
• Known allergy to vaccines or vaccine ingredients, and serious adverse reactions to vaccines, such as urticaria, dyspnea, angioneurotic edema;
• 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;
• Participated in other clinical trials prior to enrollment and during the followup 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.