Background: All of the major antigenic changes in influenza A(H3N2) viruses since 1968 have involved mutations at just nine amino acid positions, called cluster transition sites, surrounding the receptor binding region of the hemagglutinin surface protein. During the northern hemisphere (NH) 2024–2025 influenza season, A(H3N2) variants emerged with multiple parallel substitutions affecting cluster transition sites 135, 145, 158 and/or 189, with implications for the 2025–2026 season. Methods: Using >24,000 global A(H3) sequences between September 2024 and August 2025, we assess the nature and frequency of amino acid mutations among emerging NH and southern hemisphere (SH) A(H3N2) variants relative to the 2024–2025 subclade J and updated 2025–2026 subclade J.2 vaccine reference strains. We contextualize based upon historic amino acid variation among >210,000 global A(H3) sequences since 1968 and publicly available antigenic characterization data relative to 2024–2025 and 2025–2026 vaccine reference strains Results: Virtually all circulating A(H3N2) viruses in 2024–2025 were subclade J.2. In Europe, about one-third had cluster transition site mutations, mostly T135A with S145N. In North America, more than two-thirds had cluster transition site mutations, including T135K or S145N, with late-season increase in doubly mutated J.2.3 (N158K + K189R) and J.2.5 (S145N + N158K) subclades. The SH 2025 season showed increase in J.2.3 and emergence of J.2.4 (T135K + K189R) including a further drifted J.2.4.1 variant with additional N158D and other mutations, recently renamed subclade K. A substantial proportion of J.2.3 and J.2.4 viruses are antigenically distinct from the 2025–2026 influenza vaccine. Conclusion: Influenza A(H3N2) variants with a combination of cluster transition site mutations emerged during the NH 2024–2025 season. A further drifted and vaccine-mismatched variant now called subclade K arose during the SH 2025 season and is projected to predominate among A(H3N2) viruses for the NH 2025–2026 season. While mismatched vaccines may still provide protection, enhanced genetic, antigenic and epidemiological (eg, vaccine effectiveness) monitoring are warranted to inform risk assessment and response.

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