Evaluation of the epidemiological outlook of the influenza A/H3N2 clade K in England during the 2025-26 season

Key findings

• England is currently experiencing a high growth rate of infections caused by the influenza A/H3N2 K clade. Antigenic change from the previously dominant clade, a rapid selective sweep evident in genomic data, and an unusually early start to the season have raised concerns about the potential severity of this year’s influenza season.

• Analysis of publicly available data sources from the current season suggests that the effective reproduction number has been largely consistent with previous severe seasons. However, the modelled peak growth rate in the current season to date is slightly higher than previous peak growth rates from the past 10 years when subsetting for A/H3N2 cases, but comparable when aggregating all influenza cases.

• Scenario analyses using an age-stratified compartmental model compared to the previous A/H3N2 season in 2022/23 suggest that substantial immune escape is unlikely given current epidemiological trends. Current trends are compatible with small levels of immune escape in all ages, or slightly greater immune escape in children, or a 10-20% higher R0, or an earlier seed date with no change in virus fitness or immune escape. In almost all scenarios, an earlier and faster epidemic growth rate leads to depletion of susceptibles before the Christmas period with a dampening effect due to the half term school holiday.

• To support understanding and exploration of model outputs, an interactive visualisation tool was devised and made available online: https://hay-idd.shinyapps.io/ModelFluUk-H3N2/ 

• This rapid analysis is intended to support situational awareness. It provides quantitative comparisons of early epidemic growth rates with previous seasons and qualitative insights into plausible epidemic dynamics.