Routine annual influenza vaccination is recommended for all persons aged≥6 months who do not have contraindications. For those aged 6 monthsthrough 8 years who have previously received ≥2 total doses of trivalent orquadrivalent influenza vaccine ≥4 weeks apart, they require only 1 dose ofinfluenza vaccine. For those who have not previously received ≥2 doses oftrivalent or quadrivalent influenza vaccine, they require 2 dose of influenzavaccine. but the evidence on how to select vaccine doses for quadrivalentinfluenza vaccine is limited in China. The study is a prospective, open-labelcomparison of the immunogenicity and reactogenicity of 1 versus 2 doses ofan inactivated quadrivalent influenza vaccine in subjects of 3-8 years old withdifferent history of influenza vaccination.

Subjects receive 2 dosed of quadrivalent influenza vaccine 4 weeks apart.Blood samples were obtained from children at 3 time points-before receiptof dose 1, 4 weeks after receipt of dose 1 and before dose 2, and 4 weeksafter dose 2. Hemagglutination inhibition (HAI) antibody titers to the A/H3N2,A/H1N1, and B antigens included in the vaccine were measured at each timepoint

Inclusion Criteria:

• Aged 3-8 years old
• Healthy subjects judged from medical history and clinical examination
• Subjects themselves or their guardians able to understand and sign theinformed consent
• Subjects themselves or their guardians can and will comply with therequirements of the protocol
• Subjects have received ≥2 doses of trivalent or quadrivalent infuenzavaccine before enrollment (Doses need not have been received duringsame or consecutive seasons); Subjects have not received infuenzavaccine before enrollment
• Subjects with temperature <=37.0°C on axillary setting

Exclusion Criteria:

• Any prior administration of other research medicine/vaccine in last 30 days
• Any prior administration of influenza vaccine in last 6 month
• Any prior administration of immunodepressant or corticosteroids in last 3 months
• Any prior administration of blood products in last 3 months
• Any prior administration of any attenuated live vaccine in last 14 days
• Any prior administration of subunit or inactivated vaccines in last 7 days
• Subject who developed guillain-Barre syndrome post influenza vaccination
• Subject who is allergic to any ingredient of the vaccine
• Subject with acute febrile illness or infectious disease
• Thrombocytopenia, blood coagulation disorder or bleeding difficulties withintramuscular injection
• Subject with damaged or low immune function which has already beenknown
• Subject with congenital heart disease or other birth defects unsuitable for vaccination.
• Subject with respiratory diseases (including pneumonia, tuberculosis, severe asthma, etc.), heart, liver and kidney diseases, mental disorders, or chronic infections.
• Any medical, psychological, social or other condition judged byinvestigator, that may interfere subject's compliance with the protocol