How has influenza A(H3N2) subclade K spread geographically since autumn 2025 and what is its current prevalence?

1. How it appeared and where the surge began

Public health reporting and peer-reviewed surveillance trace the earliest conspicuous rise of subclade K to the southern hemisphere winter of 2025, with Australia and New Zealand showing late, extended A(H3N2) outbreaks from August that were genetically and antigenically distinct from the vaccine strain ^{[2] [1]}. WHO and national labs flagged the rapid increase in sequences assigned to the new branch beginning in August–September, which coincided with earlier-than-usual influenza activity in parts of east Asia and the Pacific ^{[1] [7]}.

2. Geographic spread since autumn 2025 — a fast, global walkaround

Genomic surveillance data and agency assessments show a fast global dispersal after the southern-hemisphere signal: WHO reported detections in more than 34 countries over six months and noted broad increases across regions except South America so far ^[1]. The ECDC concluded the subclade was detected on all continents and accounted for roughly a third of all A(H3N2) GISAID sequences between May and November 2025, with near‑half representation within the EU/EEA — evidence of widespread establishment rather than isolated importations ^{[3] [8]}.

3. National snapshots and prevalence metrics

Country-level sequencing confirms dominance where surveillance is dense: England reported 87% of A(H3N2) being subclade K between weeks 35 and 43, the CDC found 89% of characterised U.S. H3N2 viruses collected since late September were subclade K, and WHO/Europe data described up to 90% of confirmed cases in parts of the region driven by subclade K ^{[4] [5] [6]}. These figures reflect the high proportion among genetically characterised samples but should be interpreted with sampling caveats described by the agencies ^[3].

4. What prevalence numbers mean — strengths and limits of the evidence

Sequencing and sentinel surveillance give a clear signal of rapid replacement of other H3N2 sublineages in many countries, yet global prevalence estimates are constrained by uneven sequencing, reporting lags, and geographical gaps (for example, lower detection in South America) that agencies explicitly note ^{[3] [1]}. The ECDC and WHO base summaries on sequences deposited to GISAID and national surveillance streams; where sequencing is limited, local prevalence could be under- or over-estimated relative to these datasets ^[3].

5. Context: severity, vaccine match, and the media narrative

Multiple technical reports and expert commentaries stress that rapid spread does not necessarily equal greater intrinsic severity; available clinical data from east Asia and vaccine-effectiveness assessments indicate hospitalisation protection is still meaningful even if antigenic drift may reduce protection against milder infection ^{[3] [9] [4]}. Media outlets have used labels like “super flu,” a framing scientists caution is inaccurate; agencies instead emphasise monitoring, vaccination, and treatment preparedness ^{[7] [10]}.

6. Bottom line and where uncertainty remains

Since autumn 2025 subclade K has gone from a southern‑hemisphere signal to dominant—or near‑dominant—status in many regions: a third of global A(H3N2) sequences May–Nov 2025, roughly half of EU/EEA sequences, and very high proportions in surveillance-rich countries such as the UK and US ^{[3] [8] [4] [5]}. Remaining uncertainties arise from uneven genomic coverage, possible surveillance biases, and evolving real‑world vaccine effectiveness; WHO, ECDC and national agencies continue to monitor sequence data and clinical indicators ^{[1] [3] [4]}.