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

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.