An expert has urged everyone eligible to get a vaccine 'sooner rather than later' as H1N1 - known as swine flu - is circulating in the UK, with cases rising. H1N1 caused an epidemic in the UK in 2009-10 and killed 457 people.
Virus has mutated, raising vaccine questions
Simon Clarke, Associate Professor in Cellular Microbiology at the University of Reading, said: "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."
The latest data from the UK Health Security Agency (UKHSA) shows more flu activity in September than in the same month of the past four years. Professor Clarke said: "If cases continue to rise over the coming weeks, this would suggest the flu season is starting earlier than usual."
UKHSA monitoring the drifted strain
Figures from the UKHSA's annual report suggest excess deaths from flu were 3,798 last winter. A spokesman for the UKHSA said: "So far this season, there's a mix of two flu A subtypes, H1N1 and H3N2, circulating in England. The flu vaccine is designed to protect against all three 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."
They added: "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."
Experts advise getting vaccinated early
Dr Kim Goldin, senior GP and health assessment doctor at Nuffield Health, said: "The influenza virus can have a significant impact on your health and can make people really ill. It can be extremely life-threatening in a vulnerable person."
Dr Goldin added: "It can take 10 to 14 days after a vaccination for your body to build up full protection and immunity, so ideally you want to get them done in autumn so your body has time to build up protection against these viruses before the winter months."
Who is eligible for the free NHS flu vaccine
According to the NHS website, you can get the free NHS flu vaccine if you: are aged 65 or over (including those who will be 65 by March 31, 2027); are aged 18 to 64 and have certain long-term health conditions; are pregnant – you can have a flu vaccine during any stage of pregnancy; live in a care home; are the main carer for an older or disabled person, or receive a carer’s allowance; live with someone who has a weakened immune system; are aged 16 or over and rough sleeping or staying in a homeless hostel or night shelter; or are a frontline health and social care worker.
“Most school-aged children will also be offered a flu vaccinations,” adds Goldin. You can also pay to get a flu vaccination privately at a community pharmacy or private clinic even if you do not qualify for a free NHS jab. Dr Goldin said: “Viruses do change and mutate, so the flu vaccine formula is always updated every year. So, if you are eligible, you need to have it every single year.”
Flu season may be starting early
Flu infections usually begin to climb in October and peak around January. Professor Clarke told The Conversation: "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. 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."
He added: "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.
How to tell the flu from a cold
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. 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.
Professor Clarke said: "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."
