The NHS has issued an urgent warning ahead of what doctors are predicting to be the UK's worst flu season in decades.
It appears that a flu virus which mutated in the summer has started a month earlier that normal, which suggests that it could be more severe.
With the worst of winter only weeks away, leading health experts have warned that the new flu mutation could be deadly for those who are more vulnerable.
"Flu has hit early this year and will only spread in the coming weeks," Dr Suzanna McDonald, National Lead for the Influenza programme at UKHSA, said.
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"Many people often think they have flu when they only have a bad cold. If you get flu, it is nasty and for some more vulnerable it can be deadly."
Prof Nicola Lewis, the director of the World Influenza Centre at the Francis Crick Institute, said to the BBC that she wouldn't be shocked if this was the worst flu in years.
"We haven't seen a virus like this for a while, these dynamics are unusual," Prof Lewis admitted.
"It does concern me, absolutely. I'm not panicking, but I am worried."
A 'flu jab SOS' warning has been also issued by the NHS, who is making 2.4 million vaccination slots available this week.
They say it's enough to vaccinate the whole of Greater Manchester or Botswana, as eligible adults have been urged to have the jab.
Scientists constantly monitor influenza viruses because they tend to mutate, requiring yearly updates to the seasonal flu vaccine.
This particular viral evolution follows a pattern called 'shift and drift'.
Typically, the virus changes gradually through minor mutations, but occasionally undergoes a major genetic shift, creating a significantly different strain.
Such a shift occurred in June this year, when seven mutations appeared in a strain of the H3N2 seasonal flu.
UK Health Security Agency data shows 7,757 flu deaths in winter 2024–25, up from 3,555 the previous year.
Australia, meanwhile, is often a predictor for UK flu trends and it's just experienced its largest flu season on record.
Prof Derek Smith, director of the centre for pathogen evolution at the University of Cambridge, told the BBC that we are 'miles ahead' of where we'd be flu-wise and that he thinks 'it's going to be a strong flu season'.
Public Health Minister Ashley Dalton said: “Vaccination is the best form of defence against flu – particularly for the most vulnerable.
“With flu cases already triple what they were this time last year, I urge everyone eligible to take up one of the 2.4 million appointments available next week.
“Getting vaccinated now means you’ll have the best possible protection in place for yourself and your loved ones, and will also be protecting our NHS as we approach the challenging winter months.”
You can access more information on recommended vaccinations at nhs.uk/vaccinations.
Anish Vij