This is a single-center, randomized, open-label trial designed to explore the interaction between mucosal microbiota colonization in the nasal cavity and gut and the immune response to an intranasal live attenuated influenza vaccine (LAIV). The study plans to enroll 200 children and adolescents aged 6-17 years, with approximately 50% in the 6-11 years age group and 50% in the 12-17 years age group. Participants and their guardians must be able and willing to comply with the clinical trial protocol and provide informed consent. Eligible participants will be randomly assigned in a 1:1 ratio, stratified by age groups, to either the immediate vaccination group or the delayed vaccination group. Nasal swabs will be collected for all participants of both the immediate vaccination group or the delayed vaccination group after randomization for the detection of nasal and gut microbiota. Then, the immediate vaccination group will receive one dose of 0.2 mL of LAIV 28 days after randomization, while the delayed vaccination group will receive one dose of 0.2 mL of LAIV 56 days after randomization. Blood and mucosal samples (nasal stool) will be collected on the day before vaccination and on 28 days after vaccination for humoral and mucosal immunogenicity analysis. Moreover, the occurrence of adverse events within 28 days after vaccination will be collected.
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The responses of sIgA antibodies against the vaccine strain in the nasal mucosal secretions 28 days post-vaccination.
Timeframe: Day 28 post-vaccination
Changes in the density of nasal mucosal bacterial colonization (CFU/mL) compared to baseline
Timeframe: Day 28 post-vaccination
Changes in the abundance (Ξ± Diversity and Ξ² Diversity) of nasal mucosal bacterial colonization compared to baseline
Timeframe: Day 28 post-vaccination