This is a randomized, double-blinded, phase ⅠV clinical trial of SARS-CoV-2 inactivated vaccine manufactured by manufactured by Sinovac Research & Development Co., Ltd. The purpose of this study is to evaluate the safety and immunogenicity of booster vaccination with high-dosage SARS-CoV-2 vaccine in populations who have received two-dose Coronavac with 2nd dose vaccinated more than 6 month.

This study a randomized, double-blinded, phase ⅠV clinical trial of SARS-CoV-2 inactivated vaccine .The purpose of this study is to evaluate the safety and immunogenicity of booster vaccination with high-dosage SARS-CoV-2 vaccine in populations who have received two-dose Coronavac with 2nd dose vaccinated more than 6 month.A total of 1200 subjects who have received two-dose Coronavac with 2nd dose vaccinated more than 6 month will be enrolled.All of subjects will be randomly divided into two groups in a 1:1 ratio .Subjects in experimental group will receive high-dosage SARS-CoV-2 inactivated vaccine.Subjects in control group will receive medium-dosage SARS-CoV-2 inactivated vaccine .

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.