This study is a Multicenter, Randomized, Double-Blind, Placebo-Controlled, Phase Ⅲ Trial to evaluate the efficacy, safety and immunogenicity of a single dose of Live-Attenuated influenza Vaccine(LAIV) among healthy children and adolescents aged 3-17 years.

This study is a Multicenter, Randomized, Double-Blind, Placebo-Controlled, Phase Ⅲ Trial to evaluate the efficacy, safety and immunogenicity of a single dose of Live-Attenuated influenza Vaccine(LAIV) among healthy children and adolescents aged 3-17 years old. The subjects will be randomized in a 1:1 ratio of LAIV to placebo. Each subject will be vaccinated with a single dose of LAIV or Placebo. For evaluation of efficacy, subjects meeting the protocol-defined clinical case definition the 14th day post-vaccination will have a nasal swab collected for testing by RT-PCR or virus culture for evidence of influenza virus infection. All adverse events will be collected 30 minutes, 0-30 days after vaccination and all serious adverse events during the entire trial period. Blood samples will be collected from a part of subjects before vaccination and at the 30th day after immunization. Serum samples will be centrifuged for detection of antibodies to H1N1, H3N2, and influenza B.

Inclusion Criteria:

1. Healthy children and adolescents aged 3-17 years old.
2. Informed consent obtained and signed by subjects or legal guardians prior to screening. Children or adolescents aged 10-17 yrs need sign informed consent by themselves.
3. Willing to participate for the planned duration of the study, including availability for follow-up.

Exclusion Criteria:

1. Any investigational or unregistered product (drug, vaccine or device) was used within 30 days, or planned to be used during the trial.
2. have been vaccinated with any influenza vaccine within 6 months, or plan to be vaccinated during the trail.
3. Immunosuppressive agents or other immunomodulatory drugs (defined as continuous use for more than 14 days) were used over a 3-month period.
4. Immunoglobulins and / or any blood products were used within 3 months, or planned to be used before blood collection.
5. History of Guillain-Barré syndrome; History of hypersensitivity to any component of the LAIV, including egg or egg products.
6. Severe allergic reactions after vaccination (including anaphylactic shock, allergic laryngeal edema, allergic purpura, local allergic necrosis).
7. Acute diseases , infections or febrile diseases (axillary temperature ≥37.1℃) on the day of vaccination.
8. Obvious coagulation dysfunction or History of anticoagulant therapy
9. Aspirin is being used(Salicylates are a potential risk factor for Reye syndrome)
10. Known or suspected immune deficiency diseases or immunosuppressed
11. Heart disease, respiratory diseases (including severe rhinitis, nasal deformities, polyps, etc.), liver disease, kidney disease, mental disorders, chronic infections etc.