Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. For those aged 6 months through 8 years who have previously received ≥2 total doses of trivalent or quadrivalent influenza vaccine ≥4 weeks apart, they require only 1 dose of influenza vaccine. For those who have not previously received ≥2 doses of trivalent or quadrivalent influenza vaccine, they require 2 dose of influenza vaccine. but the evidence on how to select vaccine doses for quadrivalent influenza vaccine is limited in China. The study is a prospective, open-label comparison of the immunogenicity and reactogenicity of 1 versus 2 doses of an inactivated quadrivalent influenza vaccine in subjects of 3-8 years old with different 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 receipt of dose 1, 4 weeks after receipt of dose 1 and before dose 2, and 4 weeks after dose 2. Hemagglutination inhibition (HAI) antibody titers to the A/H3N2, A/H1N1, and B antigens included in the vaccine were measured at each time point

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 the informed consent
• Subjects themselves or their guardians can and will comply with the requirements of the protocol
• Subjects have received ≥2 doses of trivalent or quadrivalent infuenza vaccine before enrollment (Doses need not have been received during same or consecutive seasons); Subjects have not received infuenza vaccine before enrollment
• Subjects with temperature <=37.0°C on axillary setting

Exclusion Criteria:

• Subject who has a medical or family history of any of the following: allergic history, seizure, epilepsy, brain or mental disease
• Subject who is allergic to any ingredient of the vaccine
• Subject with damaged or low immune function which has already been known
• Subject with acute febrile illness or infectious disease
• Major congenital defects or serious chronic illness, including perinatal brain damage
• Thrombocytopenia, blood coagulation disorder or bleeding difficulties with intramuscular injection
• Subject who has serious allergic history
• Subject who developed guillain-Barre syndrome post influenza vaccination
• Any prior administration of immunodepressant or corticosteroids in last 6 months
• Any prior administration of influenza vaccine in last 6 month
• Any prior administration of blood products in last 3 months
• Any prior administration of other research medicine/vaccine in last 30 days
• Any prior administration of any attenuated live vaccine in last 14 days
• Any prior administration of subunit or inactivated vaccines in last 7 days, such as pneumococcal vaccine
• Any medical, psychological, social or other condition judged by investigator, that may interfere subject's compliance with the protocol or signature on informed consent for subject's guardian