A Clinical Trial to Evaluate the Protective Efficacy and Safety of a Nasal Spray Live Attenuated Influenza Vaccine (LAIV) in Adults Aged 18-59 Years Post-Vaccination

Inclusion Criteria:

1. Healthy participants aged 18-59 years
2. Capable of providing informed consent in person
3. Willing and able to comply with all clinical trial requirements

Exclusion Criteria:

1. Axillary temperature >37.0°C on enrollment day *
2. Received any influenza vaccine within the past 6 months or plans to receive other influenza vaccines during the trial
3. History of laboratory-confirmed influenza (by clinical, serological, or microbiological methods) within the past 6 months
4. Female participants of childbearing potential with positive urine pregnancy test, currently breastfeeding, or planning pregnancy within 6 months
5. Acute phase of any infectious disease/chronic illness within 3 days, or recent use (≤3 days) of antipyretics/analgesics/antihistamines *
6. History of severe allergies requiring medical intervention (e.g., anaphylaxis, angioedema, Arthus reaction) or hypersensitivity to vaccine components (egg proteins, gentamicin sulfate, etc.)
7. Known malignancies, autoimmune diseases (e.g., SLE, rheumatoid arthritis), or immunodeficiency (HIV, organ transplantation, etc.)
8. Asplenia, functional asplenia, or any splenectomy history
9. Use of immunosuppressants/immunomodulators (e.g., systemic corticosteroids >14 days) within 6 months or planned use during study
10. Congenital anomalies affecting organ function, uncontrolled hypertension (≥140/90 mmHg despite treatment), or severe hepatic/renal diseases (e.g., diabetes-complicated)
11. Active asthma or clinical remission for <12 months
12. History or family history of neurological/psychiatric disorders (e.g., epilepsy, Guillain-Barré syndrome)
13. Salicylate use (e.g., aspirin) within 30 days or planned use within 6 weeks post-vaccination
14. Nasal abnormalities potentially affecting vaccine administration (e.g., active allergic rhinitis with medication, ongoing nasal sprays)
15. Blood products/immunoglobulin administration within 3 months or planned use during trial
16. Live-attenuated vaccines within 14 days or subunit/inactivated vaccines within 7 days prior to enrollment *
17. Participation in other clinical trials within 1 month or concurrent involvement in interventional studies
18. Any condition deemed by investigators to compromise trial integrity