The UK Health Security Agency (UKHSA) has highlighted that influenza levels during September – while still low - were higher than in the previous four years, raising the prospect of an early ‘flu season.
UKHSA has published the first of its weekly winter respiratory virus surveillance reports, based on a range of indicators provided by GPs, hospitals, laboratories and the wider community.
The surveillance report for week 40 (28 September and 4 October 2026) shows that influenza activity increased and is circulating at baseline levels, while positivity increased with a rate of 6.5% compared with 5% on the previous week.
Influenza activity throughout September has been higher than in the same period during all past four winter seasons. While still at baseline (very low) levels, trends have been rising since August and there are possible signs that we may see an early influenza season.
The report also shows:
- the increases in influenza are being driven by young adults
- COVID-19 activity is increasing and above baseline, although still at low levels
- the strains currently circulating, including those known as XFG, are closely related to strains seen here for over a year
- Rhinovirus activity, otherwise known as the cold common, has now started to decline after a sharp rise in activity in previous weeks
- Respiratory Syncytial Virus (RSV) activity remains at baseline levels with very low levels of the virus circulating
Influenza viruses change every year, which is why vaccines are updated annually. Some years ‘drifted’ influenza viruses occur, which means the virus that is circulating has continued to evolve and has some genetic changes from the one that was used to make the vaccine.
A recent World Health Organization (WHO) technical report has indicated that one of the influenza A(H1N1) viruses that has recently been seen at low levels in Europe, and recently in the UK, has ‘drifted’ compared to the virus component used in this year’s vaccine. Last year we also saw a ‘drifted’ A(H3N2) influenza virus, but the vaccines still gave good protection.
It is still too early in the season to say with any certainty whether this drifted A(H1N1) virus will circulate widely in the UK or how effective the vaccine will be against it. Influenza is still at very low levels, but the subtyping data available so far this season shows there is an equal mix of subtypes A(H3N2) and A(H1N1) in England.
As we saw with last year’s A(H3N2) virus, which dominated, vaccines typically remain effective at preventing severe influenza, even when drifted strains are seen.
Despite the drifted virus and an earlier start to the flu season last year, the overall level of cases over the course of the season was lower and with lower mortality estimates than the previous year. Modelling indicated that, in England last winter, the vaccination programme prevented around 100,000 hospital admissions and 7,000 deaths.
The composition of flu vaccines, developed by the WHO are adjusted each year, to account for any genetic changes and are designed to help protect against the viruses that are likely to be circulating during the forthcoming flu season. Decisions about which virus strains should be included are made several months before flu season begins, so manufacturers have time to produce enough stocks, but viruses can continue to change. This means that sometimes, by the time vaccines are delivered they aren’t always an exact match against the strains circulating.