Early signs suggest that flu season may be starting earlier this year. According to our winter virus surveillance data, flu activity throughout September has been higher than in the same period over the past 4 winters. While levels remain at baseline, meaning still relatively low overall, the upward trend - if it continues in the coming weeks - suggests an early flu season. The rise so far has been driven largely by cases in the 18- to 24-year-old age group.
What strains are circulating this year?So far this season, there's a mix of 2 flu A subtypes, H1N1 and H3N2, circulating in England. The flu vaccine is designed to protect against all 3 main types of flu: influenza A(H1N1), A(H3N2) and influenza B. Whichever ends up dominating this winter - or if they all circulate at similar levels throughout - the vaccine should offer broad protection.
It might help to think of the flu vaccine like packing for a winter trip: you bring layers for rain, snow and wind because you don't know exactly what the weather will throw at you. The vaccine works the same way, offering layered protection against all 3 flu types so you're prepared, whatever ends up circulating.
What is a 'drifted' strain?Flu viruses are constantly changing through a natural process called antigenic drift. Each year, the World Health Organization monitors circulating strains globally and selects which ones should be targeted by that season's vaccine, but this decision has to be made months in advance, to give manufacturers time to produce enough doses. Sometimes the virus continues to change after that point, resulting in it no longer matching the vaccine strain as closely as originally predicted.
This year, it's the A(H1N1) subtype that has drifted from the strain used in the vaccine. It's still too early in the season to know for certain whether this drifted H1N1 will circulate widely in the UK, or how it will affect vaccine effectiveness.
How does this compare to last winter?Last winter, a drifted strain of H3N2, known as subclade K, emerged after the vaccine composition for the Northern Hemisphere had already been finalised. Despite this, the vaccine was effective and even with an earlier start to the flu season, overall levels of activity and mortality were lower than the previous year. Modelling suggests the vaccination programme in England prevented around 100,000 hospital admissions and 7,000 deaths that winter.
Does the vaccine still work against drifted strains?Drift is a normal and expected part of how flu viruses behave, and experience shows that vaccines typically remain effective at preventing severe illness even when the circulating strain doesn't perfectly match the one used to make the vaccine. Our analysis from last season found vaccinated children were around 65 to 70% less likely to attend or be admitted to hospital with flu, while vaccinated adults were around 20 to 30% less likely.
Does this affect when I should get vaccinated?Because flu activity is already trending higher than usual for this point in the year, and because it takes a couple of weeks for the vaccine to provide full protection, getting vaccinated sooner rather than later gives you the best chance of being protected before flu takes hold more widely. If this does turn into an early season, waiting could mean you're exposed during the period before your immunity has had time to build.
What's UKHSA doing to keep track of this?UKHSA is continuing to monitor the drifted H1N1 strain through laboratory analysis, routine surveillance of flu's impact on health, and ongoing studies into how well the vaccine is working in practice. This monitoring will continue throughout winter, including watching for any other strains that start to become more prominent.
What should I do now?If you're eligible for a free flu vaccine, the advice is simple: don't wait. You can check your eligibility and book an appointment through the NHS App, online, or by contacting your GP practice or a local pharmacy. For school-aged children, most vaccines are delivered in schools by local immunisation teams. Look out for consent forms and return them.
Community clinics are available for children who miss school sessions. From 1 December 2026, school-aged children from Reception to Year 11 who did not get vaccinated in school can also get vaccinated at a pharmacy.
Given the early signs this season, getting protected now, rather than later, is the best way to guard against serious illness this winter.
seen at 16:32, 8 October in UK Health Security Agency.