Recurring Temporal Association Between Influenza A Epidemics and Subsequent Influenza B Dominance in Post‐Pandemic Korea, 2022−2026

ABSTRACT Post‐pandemic influenza dynamics have shown unusual patterns globally. We investigated the re‐emergence and recurrent seasonal dominance of influenza B in Korea across three post‐pandemic seasons (2022−2026). We analyzed 157 consecutive weeks of national sentinel surveillance data (K‐SenS, Korea Disease Control and Prevention Agency), covering September 2022 to August 2025, plus preliminary data through Week 10 of 2026. Influenza B predominance was defined as weeks with > 50% influenza B among total influenza detections; a sensitivity analysis required overall influenza activity > 5%. Influenza B activity remained minimal during the 2022−23 season (mean proportion 3.3%; peak 25.3%) and never reached predominance. Two complete predominance episodes followed influenza A epidemics: after the 2023−24 epidemic (co‐circulating A/H1N1pdm09 and A/H3N2; peak ILI 61.3/1000), B predominance began January 2024 and persisted 23 weeks (peak 100%); after the 2024−25A/H1N1pdm09‐dominant epidemic (peak ILI 99.8/1000), it began February 2025 and accumulated 20 weeks over two periods with a brief interruption (peak 100%). Onset occurred 5−8 weeks after the influenza A peak by ILI (6−8 weeks virologically) in all three episodes, unaffected by threshold choice. A third episode following the 2025−26A/H3N2 epidemic was ongoing as of March 2026 (onset 7 weeks after the A peak). In post‐pandemic Korea, influenza B repeatedly established seasonal dominance 5−8 weeks after influenza A epidemics across three consecutive seasons. This recurring temporal pattern may have implications for influenza surveillance, clinical preparedness, and vaccine strategy, though its underlying mechanism remains unestablished.