Epidemiologic Overview and Viral Characteristics of Influenza A(H3N2) Subclade K during the 2025–26 Influenza Season

DOI: 10.6524/EB.202602_42(4).0001

Po-Han Huang^*, Yung-Ching Lin

2026 Vol.42 NO.4

Correspondence Author： Po-Han Huang^*

Abstract：

        This review evaluates the emergence and clinical implications of the influenza A(H3N2) variant subclade K (formerly designated J.2.4.1) during the 2025–26 season. Driven by multiple genetic mutations resulting in significant antigenic drift, subclade K had rapidly spread worldwide. Its emergence as the globally dominant strain was associated with an earlier-than-usual onset of seasonal influenza activity in several countries. In Taiwan, surveillance data indicated that A(H3N2) subclade K had emerged as the dominant strain by September 2025, with the influenza season beginning earlier than in previous years.
        Although antigenic characterization and serological assays suggested a suboptimal immune response to subclade K elicited by the 2025–26 Northern Hemisphere influenza vaccine, raising concerns over a potential vaccine mismatch, preliminary real-world evidence from Europe and the United Kingdom indicated that the current vaccine maintained robust effectiveness against influenza requiring medical attention at the primary care level, and influenza-related emergency department visits and hospitalizations in both pediatric and adult populations. Furthermore, available data suggested that subclade K did not correlate with increased disease severity or resistance to standard antiviral therapeutics. In light of the influenza subclade K outbreak in Taiwan, we recommend maintaining multifaceted epidemiologic and laboratory surveillance and reiterate the critical importance of optimizing vaccination coverage.