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

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.

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.

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.

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.