Why is the mutant flu strain more dangerous than normal? And HOW does it kill? As UK braces for ‘worst flu season on record’, experts explain

A mutant strain of the flu could be behind the UK’s worst flu season ever recorded, responsible for ‘thousands’ of deaths, experts fear. 

While cases of the flu always surge in winter, UK health officials have already sounded the alarm over a spike in early cases. 

Figures show cases are already more than three times higher than the same point last year, largely driven by a surge among school-aged children. 

NHS England chief executive Sir Jim Mackey warned there was ‘no doubt’ this winter will be one of the toughest the health service has ever faced, with hospitals at capacity, rising treatment waiting lists and patients left to endure long waits. 

Experts believe this may be because the flu strain H3N2 mutated seven times over the summer, making it more severe than usual, particularly for older people.

These genetic mutations have made it ‘hotter’ and ‘nastier’, virologists warned, making it more deadly. 

It is also different to flu strains seen in the UK in recent years, meaning people are less immune if they contract it, because their body is unfamiliar with the virus. 

Australia has just endured its worst flu season on record, with over 410,000 cases — which, experts say, is often seen as an accurate predictor of what the UK can expect. 

According to Professor Derek Smith, director of the centre for pathogen evolution at the University of Cambridge, scientists received a ‘fast increase’ in reports of the mutated H3N2 virus in June. 

Professor Punam Mangtani, a professor of epidemiology and a specialist in the spread of influenza at the London School of Hygiene & Tropical Medicine, said: ‘H3N2 is less like the one seen in 2024’s flu season. 

‘This means there may be a bit less immunity already in the population, so the virus can transmit and infect people more easily and could be a reason why cases have risen earlier than last year.’

Figures released by the UKHSA last week show that of influenza viruses subtyped, the majority were H3N2.

Previous strains of H3N2 also suggest it is a more severe form of influenza.

Prof Nicola Lewis, the director of the World Influenza Centre at the Francis Crick Institute in London, told BBC News: ‘H3 is always a hotter virus, it’s a nastier virus, it’s more impactful on the population. 

“We haven’t seen a virus like this for a while, these dynamics are unusual. It does concern me, absolutely.’  

H3N2 influenza symptoms are similar to other flu strains but can be more severe, with high fever, severe body aches, and fatigue being common. 

Anecdotal reports suggest in Australia it has caused more severe body aches, longer-lasting tiredness and symptoms come on more suddenly. 

Inflammation from the flu can make it difficult for the lungs to transfer enough oxygen to the rest of the body, raising the risk of deadly complications.  

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, Professor Lewis 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.

Such measures, they said, cut the rate by at least two thirds. 

Ahead of a predicted dramatic surge in flu cases, UK health officials have issued a ‘flu jab SOS’, urging people to come forwards for a vaccine to protect against severe illness from the virus.

As per protocol, the decision on the design of the vaccine in the UK was made back in February to ensure there was enough time to produce the millions of doses necessary. 

But given the mutant variant did not appear until June, it is not expected to offer full protection against the strain which could also explain why cases have surged so dramatically so early, experts say.  

But they caution that for those eligible, it is still vital to get the vaccine because it can lessen severe symptoms. 

Dr Mangtani said: ‘Flu strains evolve rapidly, so even if you’ve had flu or the flu vaccine previously, it’s important to still get the vaccine each year, as your natural immunity may not be as effective. 

‘It’s also best to have your flu vaccination in the autumn or early winter before cases increase, particularly if you fall into one of the recommended groups.’

The annual vaccination programme in England kicked off on October 1, offering vaccines to anyone aged 65 and over, under-65s in clinical risk groups, care home residents and carers and pregnant women.

It is also available to close contacts of those who are immunosuppressed and front-line health and social care workers, as well as children.

People not in these groups can choose to get vaccinated privately at a pharmacy for around £20. 

Stephen Griffin, a professor of virology at the University of Leeds, also told the Daily Mail: ‘Flu is less infectious than Covid, so mitigations have a bigger impact.

‘We had no flu waves in 2020/2021 because of restrictions and had we vaccinated more people in 2022, we could hav avoided the large wave caused by people being more susceptible. 

‘Making infection resilient environments by taking precautions in public spaces and especially hospitals and care settings is so important.

‘Increasing vaccine uptake can prevent susceptibility to severe infection from becoming too big and it can be done.

‘An entire lineage of influenza  went extinct as a result of mitigations in place in 2020/21.’