This study is a randomized and controlled phase Ⅳ clinical trial of the COVID-19 vaccine (Vero cell), Inactivated manufactured by Sinovac Research & Development Co., Ltd. The purpose of this study is to evaluate the immunogenicity and safety of COVID-19 vaccine (Vero cell), Inactivated co-administration with EV71 vaccine

This study is a randomized and controlled phase Ⅳ clinical trial in children aged 3-5 years old. The purpose of this study is to evaluate the immunogenicity and safety of COVID-19 vaccine (Vero cell), Inactivated co-administration with EV71 vaccine.The COVID-19 vaccine was manufactured by Sinovac Research & Development Co., Ltd, and the EV71 vaccine was manufactured by Sinovac Biotech Co.A total of 520 subjects aged 3-5 years old will be enrolled.Subjects will be randomly divided into 2 groups in a ratio of 1:1.The experimental group is the combined immunization group, which will receive the first dose of COVID-19 vaccine and the second dose of COVID-19 vaccine and EV71 vaccine on day 28;Control group is the Non-combined immunization group , which will receive the first dose of COVID-19 vaccine on day 0 ，the first dose of EV71 vaccine on day 14 ，the second dose of COVID-19 vaccine on day 28 and the second dose of EV71 vaccine on day 42.

Inclusion Criteria:

• Children aged 3-5 years ;
• The subject and/or guardian can understand and voluntarily sign the informed consent form
• Proven legal identity;

Exclusion Criteria:

• History of contact with a SARS-CoV-2 infection (positive in nucleic acid test) within 14 days;
• History of multiple system inflammatory syndrome (MIS-C);
• History of hand, foot and mouth disease, herpetic angina or EV71 vaccination;
• 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 (Systemic lupus erythematosus)or immunodeficiency / immunosuppression(HIV,history after organ transplantation)；
• 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, asplenia,functional asplenia, asplenia 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 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;
• Severe neurological disease (epilepsy, convulsions or convulsions) or mental illness;
• Axillary temperature >37.0°C;
• The subjects participated in other clinical trials during the follow-up period, or will be planned 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.