P-60 – THE ‘SUPER FLU’ THAT WASN’T: THE IMPACT OF THE H3N2 SUBCLADE K ON THE SCOTTISH POPULATION, 2025/26
Seasonal influenza remains unpredictable due to continual viral evolution, particularly within A(H3N2) lineages. Novel subclades can alter transmission, severity, and vaccine effectiveness (VE), underscoring the importance of pathogen-specific surveillance. In 2025/26, Scotland and other European countries identified a new H3N2 subclade, speculatively labelled ’Super Flu’, prompting intensified monitoring.
To describe the impact of a drifted influenza A(H3N2) subclade K virus on the Scottish population over the 2025/26 winter season.
Epidemiological data including laboratory confirmed tests, hospitalisations and mortality were used to quantify severity and transmissibility. Vaccine effectiveness was estimated and compared to previous seasons (methods published elsewhere).
Activity in 2025/26 peaked earlier (week 49, 2,406 cases) compared to the previous season (week 52, 2,954 cases). 73% of detections were influenza A, not subtyped due to resource limitations. Among subtyped influenza A and influenza B specimens, H3N2 dominated in 2025/2026 (86.2%), whereas H1N1 and B co-circulated last season (43.6% and 47.3%, respectively). Through genomic sequencing, subclade K was first identified in week 34 and became the dominant strain by week 39, accounting for 93.2% of all sequenced H3 samples and 71.6% of all influenza sequences. A higher burden was observed in children (<15 years). Early Scottish estimates of VE against influenza hospitalisations were 34% overall (95% CI: 28–39%), consistent with previous seasons, with higher VE in children (61%; 95% CI: 41–74). Hospitalisations peaked at 1,054 in week 49, below the 2024/25 peak of 1,600. Influenza-attributed deaths were 466 in weeks 40-12, compared with 599 in 2024/25.
In 2025/26, H3N2 subclade K emerged early and rapidly dominated, affecting children more, possibly due to lower prior H3N2 exposure or pre-vaccination circulation. There was no evidence of increased severity or reduced vaccine effectiveness. Subclade K reshaped the season’s timing, not its severity, dispelling early ‘Super Flu’ concerns.
