2025-26年A型H3N2流感K分支流行概況與病毒特性

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

本文針對2025年新浮現之A型流感A/H3N2變異株「K分支」（subclade K，前稱J.2.4.1）進行疫情現況分析。因累積多重基因突變造成抗原微變，K分支流感病毒已在全球迅速擴散並成為主流病毒株，導致多國流感流行期較往年提早開始。依據我國監測資料，自2025年9月起A/H3N2之K分支亦成為國內主流病毒株，且我國本季亦較往年提早進入流感流行期。雖然抗原性分析和實驗室結果顯示，K分支流感病毒抗原性偏離2025–26年北半球流感疫苗株，引發保護力下降疑慮，但歐洲和英國真實世界數據證實，當季疫苗對於預防兒童及成人因流感導致的門診、急診、住院仍具良好保護力。目前各國並未發現K分支流感病毒具抗病毒藥抗藥性或導致疾病嚴重度增加的現象。面對我國K分支的流感疫情，建議應持續多元化的流行病學與實驗室監測系統運作，並強調疫苗接種的重要性。

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.