This is a randomized, double-blind, active-controlled Phase Ⅲ clinical trial, as well as an immuno-bridging clinical trial by parallel testing previous serum after primary immunization of COVID-19 vaccine (Vero cell), inactivated (Omicron variant). The main purpose of this study is to evaluate the superiority of immunogenicity against SARS-CoV-2 Omicron strain induced by one-dose booster immunization of inactivated COVID-19 vaccine (Omicron variant), developed by Sinovac Research and Development Co., Ltd.in subjects who have received two- or three-dose Prototype COVID-19 vaccine （CZ strain）, compared with one-dose booster of Prototype COVID-19 vaccine （CZ strain） in subjects who have received three-dose Prototype COVID-19 vaccine （CZ strain）, and to evaluate the non-inferiority of immunogenicity against SARS-CoV-2 Omicron strain induced by one-dose booster immunization of inactivated COVID-19 vaccine (Omicron variant), developed by Sinovac Research and Development Co., Ltd., in subjects who have received two- or three-dose Prototype COVID-19 vaccine, compared with the immunogenicity against SARS-CoV-2 Prototype strain induced by two-dose Prototype COVID-19 vaccine（CZ strain）after primary immunization.

This study is a single-center, randomized, double-blind, active-controlled Phase Ⅲ clinical trial, as well as an immuno-bridging clinical trial by parallel testing previous serum after primary immunization of COVID-19 vaccine (Vero cell), inactivated (Omicron variant). The main purpose of this study is to evaluate the superiority of immunogenicity against SARS-CoV-2 Omicron strain induced by one-dose booster immunization of inactivated COVID-19 vaccine (Omicron variant)developed by Sinovac Life Sciences Co., Ltd.in subjects who have received two- or three-dose COVID-19 vaccine（CZ strain）, compared with one-dose booster of COVID-19 vaccine（CZ strain）in subjects who have received three-dose COVID-19 vaccine（CZ strain）and to evaluate the non-inferiority of immunogenicity against SARS-CoV-2 Omicron strain induced by one-dose booster immunization of inactivated COVID-19 vaccine (Omicron variant), developed by Sinovac Life Sciences Co., Ltd, in subjects who have received two- or three-dose COVID-19 vaccine（CZ strain）,compared with the immunogenicity against SARS-CoV-2（CZ strain）induced by two-dose COVID-19 vaccine（CZ strain）primary immunization. This clinical trial consists of two stages. A total of 1750 healthy subjects will be enrolled including 1500 healthy subjects aged 18 years and older who have received 2 or 3 doses of the COVID-19 vaccine（CZ strain）（750 population were vaccinated with 2 doses and 3 doses of COVID-19 vaccine（CZ strain）respectively）in stage I of the clinical trial,250 subjects in stage Ⅱ of the clinical trial.

Stage I of the clinical trial:1500 healthy subjects （750 population were vaccinated with 2 doses or 3 doses of COVID-19 vaccine（CZ strain）respectively）who have received two or three doses of the prototype (CZ strain) COVID-19 vaccine, including 1200 subjects aged 18~59 years old and 300 subjects aged 60 years and above. Subjects in each age group will be randomly divided into experimental group and control group in a ratio of 2:1. Subjects in the experimental group will receive one dose of inactivated COVID-19 vaccine (Omicron variant), and subjects in the control group will receive one dose of COVID-19 vaccine（CZ strain）.

Stage Ⅱ of the clinical trial:Backup serum samples will be selected from 250 healthy adult subjects aged 26-45 years who received two doses of inactivated COVID-19 vaccine(CZ strain)from clinical trial of lot-to-lot consistency of an inactivated SARS-CoV-2 Vaccine(Pro-NCOV-4001).

Inclusion Criteria:

• Healthy adults aged 18 years and above;
• Subjects completed two doses of the prototype COVID-19 vaccine (CZ strain)(the interval between the first dose and the second dose was 1-2 months)or completed 3 doses of prototype COVID-19 vaccine (CZ strain)over 6 months (the interval between the first dose and the second dose was 1-2 months, and the interval between the second dose and the third dose was 6 months or more);
• The subjects can understand and voluntarily sign the informed consent form;
• Provide legal identification and vaccination certificate of prototype COVID-19 vaccine (CZ strain).

Exclusion Criteria:

• History of SARS-CoV-2 infection(laboratory confirmed);
• Close contact with a confirmed COVID-19 (nucleic acid test or antigen test positive patients)within 14 days prior to randomization;
• Received other COVID-19 vaccine in the past except for two or three doses of prototype vaccine;
• Allergy to vaccines or vaccine/placebo ingredients, and serious adverse reactions to vaccines, such as urticaria, dyspnea, angioneuroedema;
• Autoimmune disease and/or blood disease history (including but not limited to systemic lupus erythematosus, thyroidectomy, autoimmune thyroid disease, any form of malignancy, absence of spleen, functional absence of spleen, or splenectomy for any condition),patients with well-controlled type 1 diabetes can be enrolled;
• Serious chronic diseases, such as serious cardiovascular diseases, hypertension, diabetes, liver and 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;
• Long-term history of alcohol or drug abuse;
• Received blood products within 3 months prior to receiving the investigational vaccine, or planning to receive the above 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;
• 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;
• In case of fever on the day of planned trial vaccine inoculation,axillary temperature >37.0°C;
• Women who are breastfeeding, pregnant, or planning to become pregnant during the study period (based on subjects' self-reported and urine pregnancy test results);
• Participating in or planning to participate in clinical trials of other vaccines or drugs;
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.