LONDON, UK
Flu activity is rising earlier than usual in the UK, with September levels higher than the same period in each of the previous four winters.
Health officials are also monitoring a genetically drifted A(H1N1)pdm09 strain, while NHS leaders warn that the early rise could add pressure to hospitals during the winter.
UK Flu Season Shows Early Rise
The UK could be heading towards an earlier flu season after surveillance data showed influenza activity increasing since August.
The UK Health Security Agency (UKHSA) said flu activity remained at baseline levels in its latest surveillance report, meaning overall levels are still relatively low.
However, activity throughout September was higher than during the same period in each of the previous four winters.
The latest figures cover the week from 28 September to 4 October 2026. Flu positivity increased to 6.5%, up from 5% the previous week.
The overall flu hospital admission rate was 1.97 per 100,000 people during the latest reporting week.
UKHSA said the increase was being driven mainly by young adults, adding another sign that influenza transmission is beginning earlier than expected.
NHS Raises Winter Pressure Concern
The early increase has also attracted attention from NHS leaders.
Professor Frankie Swords, National Medical Director for NHS England, described the current signs as concerning because flu admissions are already relatively high for this point in the year.
Speaking at a briefing, Prof Swords said the number of flu admissions was the highest he could remember seeing at this stage of the season.
He also pointed to the fact that the increase is occurring as COVID-19 activity begins to rise in hospitals.
The concern is therefore not that the UK is currently experiencing a severe flu outbreak.
Instead, NHS officials are watching the timing of the increase because sustained growth could create additional pressure on hospitals later in the winter.
The latest UKHSA report still classifies influenza activity as baseline, despite the upward trend.
H1N1 Strain Shows Genetic Drift
Alongside the early increase in flu activity, scientists are monitoring genetic changes in an influenza A(H1N1)pdm09 virus.
UKHSA said a recent World Health Organization technical assessment found that one A(H1N1) virus recently detected at low levels in Europe and the UK had drifted compared with the H1N1 virus component used in this year’s flu vaccine.
Genetic drift is a normal feature of influenza viruses.
As viruses replicate and spread, they accumulate genetic changes. Some of these changes can alter the virus enough to affect how well antibodies recognise it.
The current concern is therefore about how widely the drifted H1N1 virus will circulate and how well the seasonal vaccine protects against it.
It is not currently evidence that the virus has suddenly become more severe or dangerous.
What the H1N1 Change Means
The H1N1 viruses identified in England this season belong to the D.3.1.1 subclade, according to UKHSA’s latest genetic surveillance.
The agency reported low genetic diversity among the A(H1N1)pdm09 viruses analyzed so far.
This means the available evidence does not show a large number of widely different H1N1 variants circulating at the same time.
The important finding is that some of the circulating viruses have genetic differences from the vaccine reference virus.
That distinction matters because the word “mutation” can imply that a virus has become more dangerous.
Genetic mutation itself is not unusual in influenza and does not automatically mean greater disease severity.
UKHSA has not reported evidence showing that the drifted H1N1 strain is causing more severe illness.
Vaccine Protection Still Matters
The discovery of a drifted strain does not mean the flu vaccine has stopped working.
Flu vaccines are updated each year because influenza viruses continue to evolve.
The vaccine used in the UK contains components intended to protect against A(H1N1), A(H3N2) and influenza B viruses.
WHO recommended an A/Missouri/11/2025-like A(H1N1)pdm09 virus for the 2026-2027 Northern Hemisphere vaccine composition.
However, vaccine strains have to be selected months before the flu season begins so manufacturers have enough time to produce and distribute vaccines. Influenza viruses can continue to change after that decision.
This means a vaccine does not always perfectly match every virus circulating later in the season.
UKHSA said it is still too early to determine how widely the drifted A(H1N1) virus will circulate in the UK or exactly how effective the vaccine will be against it.
Importantly, the agency said flu vaccines typically continue to provide protection against severe disease even when drifted strains are circulating.
Flu Hospital Admissions Under Watch
Hospital surveillance provides another important measure of the current situation.
UKHSA recorded 234 new influenza hospital admissions during the latest reporting week.
Of these, 219 were influenza A cases without a confirmed subtype, six were A(H1N1)pdm09, one was A(H3N2), and eight were influenza B.
The overall hospital admission rate was 1.97 per 100,000.
The highest hospitalization rate was recorded among people aged 85 years and older, at 11.06 per 100,000. Despite these admissions, the overall rate remains within the baseline impact range.
Intensive care and high dependency unit admissions also remained relatively low.
UKHSA recorded 24 new ICU or HDU influenza admissions, with an overall rate of 0.06 per 100,000.
These figures show why the current situation requires monitoring while also making clear that the UK is not currently facing a high-impact flu wave.
Young Adults Drive Increase
Young adults are playing a significant role in the current increase in influenza activity.
UKHSA identified young adults as the main group driving the recent rise.
This is important because changes in infection levels among younger age groups can contribute to wider community transmission.
The trend will need to be followed through October and November to determine whether the early increase develops into a sustained seasonal wave.
The current figures alone cannot establish how severe the full 2026-2027 flu season will become.
H3N2 Also Remains Under Surveillance
A(H1N1)pdm09 is not the only influenza A virus circulating in England.
The latest UKHSA surveillance shows that A(H3N2) is also circulating.
Current subtype data indicate an approximately equal mix of A(H1N1) and A(H3N2) among the influenza A viruses that have been subtyped so far this season.
This differs from the previous winter, when A(H3N2) dominated the season.
Last season’s dominant H3N2 virus also showed genetic drift, but vaccination continued to provide protection against severe influenza.
UKHSA said modelling indicated that vaccination prevented around 100,000 hospital admissions and 7,000 deaths in England during the previous winter.
No Current Antiviral Resistance Signal
Genetic surveillance is also being used to check whether circulating viruses are developing changes that could reduce susceptibility to antiviral medicines.
The latest UKHSA surveillance has not identified mutations associated with reduced susceptibility to the antivirals oseltamivir or zanamivir.
It also reported no relevant mutations associated with reduced susceptibility to baloxavir in the viruses analysed.
This is an important distinction from the genetic drift being observed in H1N1.
A virus can acquire genetic changes without those changes making it resistant to antiviral treatment.
Flu Vaccination Remains Important
Health officials are urging people who are eligible for a free flu vaccine to get vaccinated as the season begins to increase.
UKHSA’s advice is particularly relevant because flu activity has already been higher in September than during the equivalent period of the previous four winters.
The vaccine is offered through the NHS to groups at increased risk of becoming seriously ill from influenza.
The NHS states that annual flu vaccination helps protect against influenza and is offered during autumn or early winter to eligible adults and children.
People eligible for a free vaccine can check their eligibility and arrange vaccination through the NHS App, their GP practice or a local pharmacy.
Why the Current Warning Matters
The latest developments do not show that Britain is facing a dangerous new form of flu.
Instead, three separate signals are being watched closely: influenza activity is increasing earlier than usual, NHS leaders are seeing relatively high admissions for this point in the year, and genetic surveillance has identified a drifted H1N1 virus.
The combination makes continued surveillance important as the winter approaches.
The Daily Mirror report that highlighted the H1N1 development and NHS concerns can be read here: Daily Mirror report on the UK flu strain.
The key uncertainty is what happens next. UKHSA says it is too early to know whether the drifted H1N1 virus will become widespread in the UK or how much it will affect vaccine effectiveness.
For now, influenza remains at baseline levels, but the earlier upward trend means health authorities are watching the situation closely.
What to Watch in Coming Weeks
The next few weeks will provide a clearer picture of the 2026-2027 flu season.
Health officials will continue monitoring:
- Influenza positivity in community testing
- GP and primary-care surveillance
- Hospital admission rates
- ICU and HDU admissions
- Circulating H1N1 and H3N2 subtypes
- Genetic changes in circulating influenza viruses
- Evidence of changes in vaccine effectiveness
- Antiviral susceptibility
A continued rise across several indicators would provide stronger evidence that the UK is entering an early flu season.
For now, the evidence supports caution rather than alarm.
The drifted H1N1 virus is a surveillance concern, but there is no current evidence that it is causing more severe illness or that seasonal flu vaccines have stopped protecting against serious disease.
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