Experts say the UK could be at the start of the worst flu season in a decade, much worse than last year when 8,000 people died in the UK and the previous season which saw 16,000 people die. Japan has closed schools, the UKHSA is urging people to avoid contact if they have symptoms and a doctor has told people to isolate for seven days if they fall ill.

Experts say a potentially dangerous new strain of the virus has emerged and began spreading much more early than usual in the UK - appearing to be easier to catch and easier to spread. The new mutant strain is said to be 'hotter and nastier' than the usual type of flu we get in the UK.

Professor Nicola Lewis, director of the World Influenza Centre at the Francis Crick Institute, told the BBC: "We haven't seen a virus like this for a while, these dynamics are unusual. It does concern me, absolutely. I'm not panicking, but I am worried."

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Experts say there was a sudden mutation in the flu virus detected in June this year, with seven new mutations in the H3N2 variant. Professor Derek Smith, director of the centre for pathogen evolution at the University of Cambridge, told the BBC: "It almost certainly will sweep the world."

Cases of flu started to surge in the UK in September, with experts saying it appears the mutations make the new strain more easy to catch and spread. The rise in the number of flu cases has started early in a number of countries - including the UK and Japan.

Just how easily the virus spreads will be seen as people spend more time indoors this winter. Professor Lewis said, "I think it's going to be a strong flu season".

Professor Christophe Fraser, from the Pandemic Sciences Institute at the University of Oxford, said: "It's highly likely it's going to be a bad flu season and it's going to happen quite soon, we're already well into it. There are indicators that this could be worse than some of the flu seasons we've seen in the last 10 years."

There are a number of different strains of flu, and this year the UK is suffering with infections of H3N2 influenza. Prof Lewis said: "H3 is always a hotter virus, it's a nastier virus, it's more impactful on the population."

Last year, nearly 8,000 people died from flu, and in the 2022-23 flu season there were nearly 16,000 deaths.

Japan has closed schools to help contain outbreaks. Professor Lewis said: "It might all go away by next week, but I don't think it will."

The UK Health Security Agency (UKHSA) said eligible groups should make sure they get their jab on the NHS. The data suggests younger adults and school-age children are driving the early flu wave.

Dr Mary Ramsay, Director of Public Health Prog at UKHSA: “While it is still early in the season and difficult to predict with certainty which flu strains may dominate, early data on the strains circulating to date suggest that a 'drifted' H3N2 subtype may become dominant, as has been reported in other countries.

“Strains can fluctuate throughout the season causing different waves and those more prevalent earlier may change as winter progresses. The flu vaccine has a number of components protecting against the three main flu virus types and it typically provides protection against severe clinical disease even when ‘drifted’ strains are seen.

“The UK flu programme uses technologically advanced vaccines optimised for each patient group. Whatever strains do circulate here this winter, we can be confident that the vaccine will still help give some protection to those most vulnerable from developing serious illness and being hospitalised. We strongly encourage all those eligible to get vaccinated against flu as soon as possible – it remains our best defence.”

Everyone over 65, as well as pregnant women and those in at-risk groups such as people with asthma, heart disease and diabetes, are eligible for a free flu jab on the NHS. Parents are also being reminded to sign and return consent forms to schools for children to get vaccinated, while parents and carers of 2 to 3-year-olds can also book an appointment.

Cases of flu are up across all the datasets monitored by the UKHSA, particularly in children.

Dr Conall Watson, consultant epidemiologist at the UKHSA, said: “ It’s absolutely crucial that everyone eligible for a flu vaccine books their appointment – it could make all the difference in avoiding severe illness and hospitalisation. It will also help avoid them passing on flu to those around them, including grandparents and those with long-term health conditions.

“They can usually have the flu vaccine even if they have a cold, which is very common at this time of the year.”

The UKHSA advises that people with flu or Covid symptoms, including high temperature, cough and feeling tired or achy, should try to minimise contact with other people.

Dr Cheryl Lythgoe, Society Matron at Benenden Health said: “When the flu strikes, especially at this time of year, it can really knock you off your feet. With the weather changing and lack of sunlight, these illnesses feel harder to overcome. Most people take 5 to 7 days to start feeling better, but it can take up two weeks to feel fully recovered.

“Even if your symptoms start to ease and you feel more yourself, you may still be contagious for up to seven days. That’s why it’s important to take precautions, stay home, and rest until you’ve fully recovered. This helps prevent the spread of the virus and gives your body the time it needs to heal completely. If you find symptoms worsening or not going away after 7 days, contact your GP.

“And while flu is a viral illness and won’t respond to antibiotics, a small percentage of people do develop a secondary bacterial infection after flu. Therefore, seeing your GP may be required if you aren’t recovering.”