This is an open-label, randomized and controlled phase Ⅳ clinical trial of quadrivalent influenza vaccine manufactured by Sinovac Biotech Co. , the purpose of this study is to compare the immunogenicity and safety after 1 dose of quadrivalent influenza vaccine with that of 2 doses in vaccine-unprimed children aged 3-8 years old,to compare the immunogenicity and safety after 1 dose of quadrivalent influenza vaccine in vaccine-unprimed children aged 3-8 years old with that of 1 dose in vaccine-primed children and to compare the immunogenicity and safety after 2 doses of quadrivalent influenza vaccine in vaccine-unprimed children aged 3-8 years old with that of 1 dose in vaccine-primed children.

This study is open-label, randomized and controlled phase Ⅳ clinical trial of quadrivalent influenza vaccine.The purpose of this study is to compare the immunogenicity and safety after 1 dose of quadrivalent influenza vaccine with that of 2 doses in vaccine-unprimed children aged 3-8 years old,to compare the immunogenicity and safety after 1 dose of quadrivalent influenza vaccine in vaccine-unprimed children aged 3-8 years old with that of 1 dose in vaccine-primed children and to compare the immunogenicity and safety after 2 doses of quadrivalent influenza vaccine in vaccine-unprimed children aged 3-8 years old with that of 1 dose in vaccine-primed children.A total of 600 subjects including 400 vaccine-unprimed children and 200 vaccine-primed children will be enrolled,400 vaccine-unprimed children will be randomly divided into two groups in a 1:1 ratio.Subjects in group 1(experimental group 1 ) will receive two doses of quadrivalent influenza vaccine on the immunization schedule of day 0,28 and subjects in group 2 (experimental group 2) will receive one dose of quadrivalent influenza vaccine.200 vaccine-primed children (experimental group 3) will receive one dose of quadrivalent influenza vaccine.

vaccine-unprimed children

Inclusion Criteria:

• Healthy children aged 3-8 years;
• The subjects' guardians can understand and voluntarily sign the informed consent form;
• Proven legal identity.

Exclusion Criteria:

• Received any circulating seasonal influenza vaccine prior to enrollment or had an influenza vaccine schedule during the study;
• Suffering from seasonal influenza in the past 6 moths;
• 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, 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 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;
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial. vaccine-primed children

Inclusion Criteria:

• Healthy children aged 3-8 years;
• The subjects' guardians can understand and voluntarily sign the informed consent form;
• Proven legal identity;
• Received at least 1 dose of seasonal influenza vaccine in previous epidemic seasons

Exclusion Criteria:

• Received an seasonal influenza vaccine for the current epidemic season (2020-2021 epidemic season) prior to enrollment, or had an influenza vaccine schedule during the study;
• Suffering from seasonal influenza in the past 6 moths;
• 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, 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;
• Acute diseases or acute exacerbation of chronic diseases in the past 7 days;
• Axillary temperature >37.0°C;
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.