School-age Children as the Dominant Source of Inter-seasonal Influenza Circulation in the Republic of Korea, 2022–2025

Background: Influenza circulates at low levels during summer inter-seasonal periods, potentially bridging consecutive epidemics. Which age groups contribute most to this residual circulation is not well characterized. Methods: We analyzed weekly influenza detection rates from the Korea Sentinel Surveillance System (K-SenS), stratified by 6 age groups, from 2022 to 2025 (209 weeks). The inter-season was defined as epidemiologic weeks 21–39. For each age group, we calculated the mean inter-season detection rate, the residual fraction (inter-season rate/season rate), dominance weeks (weeks with the highest detection rate), lag-1 autocorrelation of weekly rates and normalized post-peak decay curves. Results: Influenza was detected during the inter-season in all age groups. Mean inter-season detection rates were highest in children 7–12 years (6.3%) and 13–18 years old (5.2%). The residual fraction was similar across all ages (19.6%–25.6%). However, children 7–12 years old had the highest detection rate in 42% of active inter-season weeks and showed the highest lag-1 autocorrelation (0.44), indicating sustained rather than sporadic circulation. At 15 weeks after the season peak, children 7–12 years of age retained approximately 55% of their peak detection rate, compared with below 10% in adults ≥50 years old. Conclusions: School-age children, particularly those 7–12 years of age, were the dominant source of inter-seasonal influenza circulation in Korea. Their dominance was driven by high seasonal burden rather than slower viral clearance. Maintaining high vaccination coverage in this age group may help reduce inter-seasonal virus reservoirs.