This study is a randomized, double-blinded, placebo-controlled, phase Ⅱb clinical trial of an inactivated SARS-CoV-2 vaccine (CoronaVac) manufactured by Sinovac Life Sciences Co. , Ltd.The purpose of this study is to evaluate the safety of the SARS-CoV-2 Inactivated vaccine in healthy children and adolescents aged 3-17 years.

This study is a randomized, double-blinded, placebo-controlled,phase Ⅱb clinical trial in children and adolescents aged 3-17 years. The experimental vaccine and placebo were both manufactured by Sinovac Life Sciences Co. , Ltd. A total of 500 subjects will be enrolled, including 100 children aged 3-5 years, 200 children aged 6-11 years and 200 adolescents aged 12-17 years. Each age group will be randomly divided into two groups in a 3:1 ratio and received two doses of experimental vaccine or placebo on on day 0 and day 28.

Inclusion Criteria:

• Healthy children and adolescents aged 3-17 years;
• The subjects and/or guardians can understand and voluntarily sign the informed consent form (For subjects aged 8-17 years, both subjects and guardians need to sign the informed consent form);
• Proven legal identity.

Exclusion Criteria:

• Travel history / residence history of communities with case reports within 14 days;
• History of contact with a SARS-CoV-2 infection (positive in nucleic acid test) within 14 days;
• Have contacted patients with fever or respiratory symptoms from communities with case reports within 14 days;
• Two or more cases of fever and / or respiratory symptoms in a small contact area of volunteers, such as home, office etc. within 14 days;
• History of SARS-CoV-2 infection;
• History of asthma, history of allergy to the vaccine or vaccine components, or serious adverse reactions to the vaccine, such as urticaria, dyspnea, and angioedema;
• Congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.;
• 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;
• Thyroid disease or history of thyroidectomy, spleenlessness, functional spleenlessness, spleenlessness or splenectomy resulting from any condition;
• 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;
• History of alcohol or drug abuse;
• Acute diseases or acute exacerbation of chronic diseases in the past 7 days;
• Receipt of blood products within in the past 3 months;
• 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;
• Axillary temperature >37.0°C;
• Already pregnant (including a positive urine pregnancy test) or are breastfeeding, planning to get pregnant within 3 months;
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.