Britain is being battered by a record-breaking surge in flu, largely driven by the mutant H3N2 strain. 

The number of hospital beds occupied by infected patients is already higher than ever for this time of year with 'no peak in sight', officials have warned. 

Mandatory mask rules have been established in certain hospitals in England amid rising infection numbers. Globally, people in Japan and parts of the US and Spain have already been urged to mask up. 

H3N2 has not been the dominant flu virus subtype in recent years, typically it is H1N1.

But the subtype mutated seven times over the summer, helping it to evade previous immunity, sparking alarm among experts.

Dubbed subclade K, it is now responsible for more than 80 per cent of flu cases, UK Health Security Agency (UKHSA) data shows. 

Yet, according to Chicago-based Dr Mary Rodgers, it is almost impossible to distinguish H3N2 from other flu strains without laboratory testing, meaning it may be circulating at even higher levels undetected. 

Equally, while experts are still calling on Britons to get the flu vaccine — the best defence against severe infection — it may not be as effective as previous years given subclade K didn't emerge until the summer. 

Dr Rodgers manages a global viral surveillance program at medical device and health tech company Abbott as part of its Pandemic Defense Coalition.

While her official title is associate research fellow, Rodgers is also known as a 'virus hunter', given her mission is to stay a step ahead of emerging viruses so the world can be prepared to combat them. 

Dr Rodgers told the Daily Mail: 'H3N2 isn't new, but when it becomes the dominant strain, we often see more severe flu seasons. 

'Recent H3N2 viruses have picked up mutations that help them spread more easily or partially evade immunity. 

'That can lead to more infections—and potentially more hospitalizations—when the strain circulates widely.'

Flu viruses mutate every year — a process known medically as antigenic drift — but scientists are still unclear why it acquired seven new mutations this summer. 

She added: 'Unfortunately, you can't distinguish H3N2 from other flu strains based on symptoms alone—they all look and feel very similar. 

The only way to know which strain is circulating is through sequencing, which is done on leftover samples by research or public health laboratories.'

Dr Antonia Ho, a clinical senior lecturer and consultant in infectious diseases at the University of Glasgow, also said: 'H3N2 tends to cause more severe illness than H1N1, particularly in older adults. 

'In the 2022/23 winter season when H3N2 was the dominant virus, there were 16,000 influenza-associated deaths, compared to 8,000 last winter.'  

The UK's flu R rate — or reproduction value — normally sits around 1.2, meaning that for every 100 people who had flu, they could pass it to roughly 120 people. 

The R rate does not directly convert into infections — but shows that the outbreak is growing if it sits above the crucial level of one. Below this, it means not everyone is passing the disease on. 

But early estimate for this year is 1.4, Dr Ho said, roughly translating to passing the virus on to 140 people if 100 had the flu. 

For context, however, at the beginning of the Covid pandemic, virology experts estimated Covid's R rate to sit between 2.4 and 4 in the UK, before dramatic stay-at-home lockdown measures were enforced.

Yet, experts have also feared the vaccine may not be as effective at protecting against the new strain.

The flu vaccine is changed each year based on data from countries in the southern hemisphere, like Australia and New Zealand, who have their winter season six months before Britain.

This season's vaccine was designed to protect against two subtypes of H1N1, influenza B, and a type of H3N2 that predates the new subclade K.

In 2017, a mismatched flu vaccine was responsible for 50,000 extra deaths in the UK, according to the Office of National Statistics. 

Data suggests the flu jab used in the southern hemisphere, which the one being rolled out in the UK is based on, could be up slightly less effective than normal. 

Experts consider a successful flu jab to be one that prevents people from needing hospital care for the virus in about six in 10 cases.

According to latest UKHSA figures the 2025/26 vaccine is currently 70 to 75 per cent effective at preventing hospital attendance in children aged 2 to 17 years and 30 to 40 per cent effective in adults.

But even in seasons when the vaccine is less effective, it still offers some protection, experts say and it is the most effective weapon for reducing the risk of serious illness.

Dr Rodgers told the Daily Mail: 'We can expect new strains to continue emerging. 

'Even with that evolution, the flu vaccine remains one of our strongest tools for preventing severe illness and complications.'

Since September, UK health officials have urged the millions eligible to come forward for their Covid and flu jabs, to reduce their risk of becoming severely unwell and needing hospital care.

People who are aged over 65, those with long term health conditions, pregnant women, care home residents and carers are eligible for a free jab.

And there are signs that calls for eligible Brits to roll up their sleeves and get jabbed have been heard, with almost 17 million vaccines now administered in England. 

It comes as surveillance programmes that monitor the UK's outbreak suggest flu hospital admissions in England are more than double last year over the same time period and ten times higher than 2023. 

Figures show an average of 1,717 beds alone were taken up by flu patients each day last week, up on the 1,098 in 2024 and 160 in 2023. 

Of these, 69 were in critical care — almost double the 39 logged last year.

NHS England chief executive Sir Jim Mackey yesterday confirmed flu hospital admissions could triple or even quadruple by next week and could require a 'national response'.

At a board meeting of NHS England, Sir Jim — who took up the role earlier this year — said: 'I think it's reasonable to assume there will be between 5,000 and 8,000 occupied beds around this time next week.' 

He also branded the looming resident doctors' strike 'cruel and calculated', warning that it was intended to cause 'mayhem' at such a perilous time. 

Thousands of medics — previously known as junior doctors — will walk out from December 17 until December 22. 

He added: 'There's a genuine and palpable feeling of anger, frustration and exasperation.

'We're all just really trying to get our heads around something that feels cruel and feels calculated to cause mayhem at a time when the service is really pulling all the stops out to try and avoid that and keep people safe.'