Data from the UK Health Security Agency (UKHSA) shows more flu activity in September than in the same month of the past four years. However, flu is still circulating at relatively low levels in the community. If cases continue to rise over the coming weeks, this would suggest the flu season is starting earlier than usual.
Flu infections usually begin to climb in October and peak in the coldest months, around January.
One of the main types circulating is H1N1, known as swine flu, which caused an epidemic in the UK in 2009-10 and killed 457 people. The H1N1 virus circulating now has undergone changes that make it different from the version targeted by this year’s vaccine. This raises questions about how well the vaccine will protect people against it.
Flu viruses change a little from year to year. These changes, known as antigenic drift, can make it harder for the immune system to recognise a virus from a previous infection or vaccination. They affect proteins on the virus’s surface that the immune system recognises, potentially making a vaccine less effective against a changed version.
Each year, scientists at the World Health Organization study which flu viruses are circulating and use this information to recommend which versions should be included in vaccines. They have to make these decisions months in advance, so manufacturers have time to produce enough doses. Sometimes the viruses that spread turn out to be different from those the vaccine was designed to target.
The UKHSA is monitoring the changed virus closely. There is not yet enough information to know whether it will spread widely, cause more serious illness or make this year’s vaccine less effective.
A drifted strain does not guarantee a bad season. The variant may not become the dominant type, and the changes may not be enough to weaken the vaccine.
Last winter offers some reassurance. A drifted strain emerged after the northern hemisphere vaccine had been finalised, yet the jab still proved effective. Despite an early start to the 2025-26 season, flu activity and death rates were lower than the year before.
In England alone, the vaccination programme is believed to have prevented about 100,000 hospital admissions and 7,000 deaths.
How to tell the flu from a cold
With flu circulating earlier than usual, it helps to know how it differs from a common cold – and why symptoms alone can’t always tell you which virus you have.
Flu, COVID and respiratory syncytial virus (RSV) share many symptoms, including fever, cough, fatigue and body aches. Telling them apart by symptoms alone is nearly impossible – although some patterns offer clues.
COVID can bring a sudden loss of taste or smell and shortness of breath, with symptoms building over a few days. Flu tends to strike abruptly, with a rapid onset of high fever, severe aches, chills and headaches.
Anyone who can turn up to work and function with a cough and runny nose probably has a cold, not flu – though their colleagues are unlikely to thank them for spreading it.
A test can help identify which virus is causing an infection. An antigen test detects proteins from the virus, while a PCR test looks for the virus’s genetic material. Both usually involve taking a swab from the nose or throat.
At this time every year, the NHS starts its flu vaccination publicity campaign; some GP surgeries send out emails and text messages offering a free flu vaccine to those who are eligible. High street pharmacies also begin advertising private jabs for a small fee, usually around £20.
If you’re eligible for a free NHS vaccine, it can be booked online. If you need to buy it privately through a high street pharmacy, it can usually be booked in branch or via their website, although stocks often sell out quickly. However you get a vaccine, it takes about 14 days to build maximum protection.
No vaccine programme is a guarantee of protection. But with flu activity rising early, it makes sense to get vaccinated sooner rather than later, so that your defences are in place before infections peak rather than while an epidemic rages.
Simon Clarke receives funding from the British Heart Foundation and Defence Science & Technology Laboratory.