This study is a randomized, double-blinded, and placebo controlled phase 1&2 clinical trial of the SARS-CoV-2 inactivated vaccine manufactured by Sinovac Research & Development Co., Ltd. The purpose of this study is to evaluate the safety and immunogenicity of the experimental vaccine in healthy children and adolescents aged 3-17 years

This study is a randomized, double-blinded, single-center, placebo-controlled phase 1&2 clinical trial in children and adolescents aged 3-17 years. The experimental vaccine and placebo were both manufactured by Sinovac Research & Development Co., Ltd. A total of 552 subjects will be enrolled, with 72 at phase 1, and 480 at phase 2. Subjects will be assigned to receive two doses of different dosage of experimental vaccine or placebo on the schedule of day 0,28. Subjects in Phase receive the second dose 10 months or 12 months after the second dose.

Inclusion Criteria:

• Healthy children and adolescents aged 3-17 years;
• The subject and/or guardian can understand and voluntarily sign the informed consent form (double sign required for 8-17 years old);
• 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;

• Physical examination has clinically significant abnormal hematology and biochemistry laboratory test results that exceed the reference value range (only applicable to phase I clinical trials):

  1. Blood routine test: white blood cell count, hemoglobin, platelet count;
  2. Detection of blood biochemical indicators: alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin (TBIL), creatinine (CR), fasting blood glucose;
  3. Urine routine index: urine protein (PRO);

• History of alcohol or drug abuse;

• 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;

• Acute diseases or acute exacerbation of chronic diseases 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.