A phase III clinical trial of the study of quadrivalent influenza vaccine developed by Sinovac Biotech will be conducted in Chinese children aged 6 to 35 months. The trial is an randomized, double-blind and active controlled study. The objective of this study is to evaluate the Immunogenicity and safety of the vaccine.

A phase III clinical trial of the study of quadrivalent influenza vaccine developed by Sinovac Biotech will be conducted in Chinese children aged 6 to 35 months. The trial is an randomized, double-blind and active controlled study. The objective of this study is to evaluate the safety and immunogenicity of quadrivalent influenza vaccine(0.25ml and 0.5ml) manufactured by Sinovac Biotech Co., Ltd in healthy infants aged from 6 to 35 months. A total of 3300 subjects will be enrolled. The subjects will be randomly assigned to 4 groups in a 2:2:1:1 ratio and subjects will receive two doses of quadrivalent influenza vaccine(0.25ml or 0.5ml) or trivalent influenza vaccine(BV or BY), respectively, according to the immunization schedule of day 0,28 in each group.

Inclusion Criteria:

• Healthy infants aged 6-35 months;
• Proven vaccination certificate and birth certificate;
• The subjects' guardians can understand and voluntarily sign the informed consent form.

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;
• Axillary temperature >37.0°C;
• History of allergy to the vaccine or vaccine components, or serious adverse reactions to the vaccine, such as urticaria, dyspnea, angioedema and asthma;
• Congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.;
• Severe chronic diseases(Such as down syndrome, diabetes, sickle cell anaemia or neurological disorders);
• Severe neurological disease (epilepsy, convulsions or convulsions) or mental illness;
• Autoimmune disease (such as systemic lupus erythematosus) or immunodeficiency / immunosuppression (such as HIV, after organ transplantation)
• 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;
• Continuous use of corticosteroids or other immunosuppressive agents for ≥ 14 days within the past 6 months (excluding allergic rhinitis corticosteroid spray therapy, acute noncomplicated dermatitis superficial corticosteroid therapy) and cytotoxic therapy in the past 6 months;
• Acute diseases or acute exacerbation of chronic diseases in the past 3 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;
• Participated in other clinical trials before enrollment and in the follow-up period, or plans to participate in other clinical trials during the clinical trial;
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial