This is a single-center, open-label, single-arm clinical trial designed to evaluate the mucosal and systemic immune responses induced by an intranasal influenza live attenuated vaccine, and to explore the correlation between the two. The study plans to recruit 60 healthy volunteers aged 14-17 years, with a male-to-female ratio of 1:1. Participants and their guardians must be able and willing to comply with the clinical trial protocol and provide informed consent. Eligible participants will receive a single 0.2 mL dose of LAIV prior to the influenza season. Information on participants' historical vaccination records, baseline demographic data, and past health status will be collected. Nasal specimens (nasal secretions and nasal mucosal cells) and blood samples (serum and PBMCs) will be collected on Day 0 (before vaccination), Day 7, and Day 28 after vaccination for transcriptome sequencing and immunogenicity analysis. In addition, fecal samples will be collected on Day 0 (before vaccination), Day 7, and Day 28 after vaccination for intestinal microbiota detection.

Inclusion Criteria:

1. Individuals aged 14-17 years.
2. Volunteers and their guardians are able and willing to comply with the requirements of the clinical trial protocol and sign the informed consent form.

Exclusion Criteria:

1. Allergy to any component of the vaccine product, including eggs, excipients, or gentamicin sulfate.
2. Individuals with acute illnesses, severe chronic illnesses, acute exacerbations of chronic illnesses, or fever.
3. Pregnant or lactating women.
4. Individuals with Leigh syndrome who are receiving treatment with aspirin or aspirin-containing medications.
5. Individuals with immunodeficiency, immunosuppression, or those undergoing immunosuppressive therapy.
6. Individuals with uncontrolled epilepsy, other progressive neurological disorders, or a history of Guillain-Barré syndrome.
7. Individuals with rhinitis or asthma.
8. Individuals with a personal or family history of seizures, chronic diseases, epilepsy, or allergic tendencies.
9. Individuals who have received immunoglobulin injections within the last 3 months prior to vaccination.
10. Individuals who have used other live attenuated vaccines within 1 month prior to vaccination.
11. Individuals planning to use antiviral drugs for influenza within 48 hours before or 2 weeks after vaccination.
12. Individuals planning to use any other intranasal medication within 2 days after vaccination.
13. Subjects with underlying gastrointestinal diseases, including but not limited to inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), chronic diarrhea, peptic ulcer, and intestinal polyps.
14. Subjects with metabolic diseases, such as diabetes mellitus, obesity (BMI ≥ 25.9 kg/m²), hyperlipidemia, or metabolic syndrome.
15. Subjects who have used antibiotics, probiotics, or other drugs that may affect the intestinal microbiota within the past 3 months.
16. Other circumstances in which the subject is deemed unsuitable for participation in the study by the investigator.