The NHS says it’s facing its “worst-case scenario” after the number of people in hospital with flu jumped by 55% in a week in England.

NHS England chief executive Sir Jim Mackey says between 5,000 and 8,000 hospital beds could be filled with flu patients by the weekend.

Health experts at the King’s Fund think tank have said talk of an “unrelenting flu wave” has become worryingly familiar over recent years. But Chris Streather, the medical director for the NHS in London, said the situation was “well within the boundaries” of what the NHS could cope with.

How then is winter 2025 really any different and which patients have been affected most by what the NHS is now describing as “super flu”?

The major difference between 2025’s flu season and the previous three years is that the virus started spreading around a month earlier.

The first sign of this was in October in data published by the UK Health Security Agency (UKHSA).

When someone goes to their GP or hospital with flu-like symptoms they can be tested for a number of viruses including influenza, Covid and RSV.

UKHSA records the percentage of those tests that come back positive for flu in England, which can then give a strong indication that rates in the community are either rising or falling.

The picture across the four nations of the UK is broadly similar with a sharp increase in positive tests for flu in Scotland, Wales and Northern Ireland.

Some virologists have linked the earlier flu season this year to a subtle shift in the genetic makeup of the main flu virus that is circulating – called H3N2.

So-called ‘super-flu’ is not a medical term and it does not mean the virus is more severe or harder to treat.

But the general public has not encountered this exact version of flu before, which means there may be less immunity built up in society, allowing it to spread more easily.

Children tend to be more susceptible to flu than older adults, partly because their immune systems are still developing and because they tend to spread viruses more quickly through close contact.

The latest breakdown of UKHSA data shows that the proportion of positive tests is currently much higher in children and young people still at school or university.

Some schools have had to bring back Covid-like measures to prevent the spread of the virus, such as cutting back on singing in assemblies and introducing sanitisation stations, while one site in Caerphilly had to close temporarily.

Each year thousands of otherwise healthy children end up in A&E with complications after catching influenza.

But there is another concern: that younger people will go home and then spread the disease to elderly relatives who tend to be more vulnerable.

The NHS records the number of patients in hospital each week with influenza and other types of respiratory illness.

The number has been rising sharply in England with an average of 2,660 flu patients taking up a hospital bed last week, up from 1,717 in the previous week.

Those over 85-years-old are five times more likely to be hospitalised than the general population.

But the patients being admitted now would have been infected with the virus a week or so ago when infection rates were lower.

The greater concern for the health service is what happens over the coming weeks as new cases appear in A&E.

The NHS has roughly 105,000 available hospital beds in England and tends to “run hot” over the winter with 95% of those taken up at any one time.

If the number of flu patients needing overnight treatment jumps to 5,000 or higher, as Sir Jim Mackey predicts, then it could put the whole hospital system under more pressure.

The message coming from doctors and the NHS is for people in vulnerable groups to continue to come forward for a flu vaccine.

Even though the genetic make-up of the virus has shifted this winter, the main jab is still thought to offer effective protection, particularly against severe disease.

The flu vaccine is free on the NHS for those over 65-years-old, young children, pregnant women, those with certain health conditions, carers, and front line health and social care workers.

People in other groups can get the same vaccine for between £15 and £25 from high street pharmacists.

As of 30 November, around 70% of older people and care home residents had taken up the offer of a free flu jab.

But vaccination rates in younger at-risk groups such as the clinically vulnerable were lower at just over 40%.

Rates among NHS workers in England, which have fallen back since the Covid pandemic, appear to have stabilised this year at roughly the same level – around 42%.