This study is a randomized, double-blinded, and placebo controlled phase 1&2 clinical trial of the SARS-CoV-2 inactivated vaccine manufactured by Sinovac Life Sciences Co. , Ltd. The purpose of this study is to evaluate the safety and immunogenicity of the experimental vaccine in healthy elderly aged 60 years and above.

This study is a randomized, double-blinded, single-center, placebo-controlled phase 1&2 clinical trial in healthy elderly aged 60 years and above. The experimental vaccine and placebo were both manufactured by Sinovac Life Sciences Co. , Ltd. A total of 422 subjects will be enrolled, with 72 in phase 1 and 350 in phase 2. 72 Subjects with 36 in medium-dosage group and 36 in high-dosage group in phase 1 will receive two doses of primary immunization according to the immunization schedule of day 0, 28 and the subjects at each dosage group will be assigned in a 2:1 ratio to receive investigational vaccine or placebo respectively.All enrolled subjects will receive 1 dose of booster immunization 1 year after primary immunization.350 Subjects in phase 2 will receive two doses of primary immunization according to the immunization schedule of day 0,28,the subjects will be assigned in a ratio of 2:2:2:1 to receive the low dosage, medium dosage, high dosage vaccine, or placebo. All enrolled subjects will received 1 dose of booster immunization(the third dose ) 6 months after primary immunization.And subjects in medium-dosage group and high -dosage group will receive the second booster dose (the fourth dose) 1 year after the second dose.

Inclusion Criteria:

• Healthy adults aged ≥60 years;
• Be able to understand and sign the informed consent voluntarily;
• Provide legal identification;

Exclusion Criteria:

• Travel / residence history of Wuhan city and surrounding areas or other communities with case reports within 14 days prior to the enrolment;
• Contact with SARS-CoV-2 infected persons (positive for nucleic acid detection) within 14 days prior to the enrolment;
• Contact patients with fever or respiratory symptoms from Wuhan city and surrounding areas, or from communities with case reports within 14 days prior to the enrolment;
• Two or more cases of fever and / or respiratory symptoms in a small contact area of subjects, such as family, office, school class or other places within 14 days prior to the enrolment;
• History of SARS;
• History of SARS-CoV-2 infection;
• History of asthma, allergy to vaccines or vaccine ingredients, and serious adverse reactions to vaccines, such as urticaria, dyspnea, angioneuroedema;
• Congenital malformation or developmental disorder, genetic defect, severe malnutrition, etc;
• Autoimmune disease or immunodeficiency / immunosuppression;
• Serious chronic disease, serious cardiovascular disease, hypertension and diabetes that cannot be controlled by drugs, hepatorenal disease, malignant tumor, etc;
• Serious nervous system disease (epilepsy, convulsion or convulsion) or psychosis;
• Thyroid disease or history of thyroidectomy, asplenia, functional asplenia, asplenia or splenectomy resulting from any condition;
• 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 non-complicated dermatitis superficial corticosteroid therapy) in the past 6 months;
• Long history of alcohol or drug abuse;
• Receipt of blood products 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;
• Receipt of inactivated or subunit vaccines in the past 7 days;
• Acute diseases or acute exacerbation of chronic diseases in the past 7 days;
• Axillary temperature >37.0°C;
• According to the investigator's judgment, the subject has any other factors that are not suitable for the clinical trial.