Influenza A and B Viral Shedding in Adults Immunized with Live Attenuated Influenza Vaccine

BACKGROUND: Live attenuated influenza vaccine (LAIV) induces mucosal immunity through limited viral replication in the upper respiratory tract, but post-vaccination viral shedding dynamics and their clinical correlates remain incompletely characterized. METHODS: We evaluated 283 healthy adults (108 in 2023-2024, 175 during the 2024-2025 influenza seasons) following intranasal LAIV administration. Nasal swabs were collected on post-LAIV days 1, 2-4, and 5-7 to quantify influenza A and B RNA by RT-PCR. Viral detection, shedding duration and burden, clearance kinetics, and probability of detection were compared across seasons, vaccine strain compositions (quadrivalent vs. trivalent), and host factors. Respiratory symptoms were assessed. RESULTS: Early viral shedding was frequent: influenza A or B RNA was detected in 86.9% of participants on day 1, with co-detection in 52.7%. Probability of detection declined from 92% on day 1 to 9% on day 7. Influenza B had longer shedding duration (median, 2.0 vs. 1.0 days; p<0.001) and higher shedding burden (4.2 vs. 4.0 log₁₀ RNA copies/mL; p<0.001). Three clearance profiles - rapid (clearance ≤4 days), moderate (5-7 days), and slow (≥7 days) - were identified. Profiles were consistent across seasons but influenza B was disproportionately represented in slower-clearance groups (p<0.001). Total viral burden (p<0.01) and shedding duration (p=0.01) were associated with total symptom burden. CONCLUSIONS: LAIV replication begins within 24 hours and declines over the first week, with persistent shedding uncommon after day 7. Influenza B replicates more extensively and persists longer than influenza A. Symptom burden correlates with shedding duration and viral burden.