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

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.