This is an open phase Ⅲ clinical trial of COVID-19 vaccine (Vero cell), inactivated manufactured by Sinovac Research & Development Co., Ltd.The purpose of this study is to evaluate Immunogenicity and safety of a third dose at different times after two-dose of COVID-19 vaccine (Vero cell), Inactivated in the Elderly Aged 60 years and above.

This study is an open phase Ⅲ clinical trial. The main purpose of this study is to evaluate Immunogenicity and safety of a third dose at different times after two-dose of COVID-19 vaccine (Vero cell), Inactivated manufactured by Sinovac Research & Development Co., Ltd. in the elderly aged 60 years and above.A total of 400 subjects aged 60 years and above who had received two doses of COVID-19 vaccine(Vero Cell), inactivated manufactured by Sinovac Research & Development Co., Ltd. between 3 - 6 months ago (100 subjects for 3 months, 4 months, 5 months and 6 months,respectively) will be enrolled and receive a third dose of two-dose of COVID-19 vaccine (Vero cell), inactivated were enrolled.

Inclusion Criteria:

• Subjects aged 60 years and above;
• Provide legal identification;
• Have completed two-dose of COVID-19 vaccine (Vero cell), inactivated manufactured by Sinovac Research& Development Co., Ltd. for 3~6 months（The interval between two doses was 21~35 days）；
• Be able to understand and voluntarily sign the informed consent form and be willing to complete the study according to the research plan.

Exclusion Criteria:

• History of SARS-CoV-2 infection(laboratory confirmed);
• Those who have received COVID-19 vaccine from other manufacturers other than Sinovac Research& Development Co., Ltd., or who have received booster immunization with COVID-19 vaccine after two doses of COVID-19 vaccine (Vero cell),inactivated manufactured by Sinovac Research& Development Co., Ltd.;
• History of serious adverse reactions to the vaccine or vaccine components, such as urticaria dyspnea and angioneuroedema;
• Autoimmune disease or immunodeficiency/immunosuppression;
• 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;
• Immunosuppressive therapy, cytotoxic therapy, inhaled corticosteroids(excluding allergic rhinitis corticosteroid spray therapy, acute noncomplicated dermatitis superficial corticosteroid therapy) in the past 6 months or plan to receive these treatments during the study period;
• Receipt of blood products within in the past 3 months or plan to receive these treatments during the study period;
• Receipt of other investigational drugs in the past 30 days;
• Receipt of attenuated live vaccines in the past 14 days,or receipt of inactivated or subunit vaccines in the past 7 days;
• Onset of various acute or chronic diseases within 7 days prior to the study;
• Axillary temperature >37.0°C;
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.