This study is a randomized, double-blinded clinical Trial, to evaluate the non-inferiority of the commercial scale Inactivated SARS-CoV-2 vaccine against that of the pilot scale among adults aged 26-45 years, and the open-labelled, bridging non-inferiority of the vaccine induced immunogenicity in elderly against that in adults.

This study is a randomized, double-blinded clinical Trial, to evaluate the non-inferiority of the commercial scale Inactivated SARS-CoV-2 vaccine against that of the pilot scale among adults aged 26-45 years, and the open-labelled, bridging non-inferiority of the vaccine induced immunogenicity in elderly against that in adults. The experimental vaccine was manufactured by Sinovac Research & Development Co.,Ltd. Totally 1040 subjects，including: 130 subjects aged 18~25 years; 520 subjects aged 26~45 years, with 260 in each group; 130 subjects aged 46~59 years; 260 subjects aged ≥60 years.

Subjects will be assigned to receive two doses of medium-dosage vaccine on the schedule of 0,14.

Inclusion Criteria:

1. Healthy adults aged 18-59 or elderly aged ≥60 years;
2. Proven legal identity.

Exclusion Criteria:

1. Travel/residence history of communities with reported cases within 14 days before trial;
2. History of exposure to Novel Coronavirus infection (nucleic acid positive) within 14 days prior to the trial;
3. Exposed to patients with fever or respiratory symptoms from communities with reported cases within 14 days prior to the trial;
4. Two or more cases of fever and/or respiratory symptoms in a small area such as home, office, school and class within 14 days prior to the trial;
5. History of SARS-CoV-2 infection;
6. History of asthma, history of allergy to vaccines or vaccine ingredients, and severe adverse reactions to vaccines, such as urticaria, dyspnea, and angioneurotic edema;
7. Congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.;
8. Autoimmune disease or immunodeficiency/immunosuppression;
9. Patients with serious chronic diseases, serious cardiovascular diseases, uncontrolled hypertension and diabetes, liver and kidney diseases, malignant tumors, etc.;
10. Serious neurological disorder (epilepsy, convulsions or convulsions) or psychosis;
11. History of thyroid disease or thyroidectomy, absence of spleen, functional absence of spleen, and absence of spleen or splenectomy caused by any circumstance;
12. Coagulation dysfunction (such as coagulation factor deficiency, coagulation disease, platelet abnormality) or obvious bruising or coagulation disorder diagnosed by doctors;
13. Immunosuppressive therapy, cytotoxic therapy, inhaled corticosteroids (excluding corticosteroid spray therapy for allergic rhinitis, and surface corticosteroid therapy for acute non-complicated dermatitis) in the past 6 months;
14. Long history of alcohol or drug abuse;
15. Received blood products within 3 months prior to receiving the vaccine;
16. Received other research drugs within 30 days prior to receiving the vaccine;
17. Received live attenuated vaccine within 14 days prior to receiving the vaccine;
18. Received subunit or inactivated vaccine within 7 days prior to receiving the vaccine;
19. Onset of various acute or chronic diseases within 7 days prior to the study;
20. Axillary temperature of >37.0℃ before inoculation of the vaccine;
21. Those who are already pregnant (including a positive urine pregnancy test) or are breastfeeding, planning to become pregnant within 2 months;
22. In the opinion of the investigator, the participants had any other factors that made them unsuitable to participate in the clinical trial.

The first part of the study is a double-blinded trial, the second part is a open-label research.