Influenza A H3N2, specifically the subclade K variant, has undergone several mutations with notable effects:

H3N2 subclade K picked up at least seven key mutations during the summer of 2025, enhancing its ability to evade immunity from existing vaccines and past infections.

It is now dominant in the UK, Japan, Canada, and spreading across all continents, representing approximately 90% of H3N2 cases in several regions. Flu seasons in the UK began 5 weeks earlier than usual, leading to higher numbers of hospitalisations.

The current vaccine targets an older H3N2 subclade (J.2), so isn’t perfectly matched to subclade K. However, it still offers good protection against severe illness. Though it may not completely prevent infection, vaccinated individuals are significantly less likely to experience hospitalisation or severe disease.

The variant is causing a “toughest winter ever” in terms of flu burden with UKHSA anticipating high flu activity, especially among children and young adults.

However, official advice has not changed:

Government advice and guidance for managing acute respiratory infections (ARIs), such as flu, has not formally changed for the adult social care sector. The focus remains on embedding good standard infection control precautions (e.g. hand hygiene, cough/respiratory hygiene, good ventilation, and the appropriate use of PPE).

The most appropriate/ up to date guidance for managing flu is at Infection prevention and control in adult social care: acute respiratory infection - GOV.UK

Read about this and more in our weekly newsletter for Wednesday 10 December.