Immunogenicity and Safety of a Third Dose of COVID-19 Vaccine(Vero Cell), Inactivated in the Elderly

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.

Eligibility Criteria:
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.