Emerging Influenza A(H3N2) Subclade K: A Narrative Review

Abstract

Viruses spread rapidly by antigenic drift, with the impetus to mismatch vaccines and early season epidemics. One new variant of Subclade K in multiple regions in late 2024 and early 2025 appeared at a rapid rate, becoming the dominant contributor to influenza activity in multiple regions. This narrative review summarizes existing evidence regarding the subclade’s viral characteristics, epidemiological behavior, clinical relevance and public health implications, from a peer-reviewed literature, genomic surveillance data, antigenic assays, and initial vaccine effectiveness estimates from ECDC and UKHSA. In addition, subclade K contains multiple hemagglutinin substitutions that reduce antigenic similarity to the 2025–2026 vaccine strain, with serological tests showing minimal cross-reactivity and suggesting immune escape. Within the EU/EEA, the subclade has risen rapidly over the last year, although there is no evidence of increased intrinsic virulence. The burden of health care could increase if early season transmission is further delayed. Early estimates of vaccine effectiveness provide strong protection in children of 72–75 per cent, with protection reduced by adults to 32–39 per cent. Subclade K is a major evolutionary event which presents the need for increased genomic surveillance, timely vaccination and coordinated public health preparedness to mitigate its present and future consequences.