Early Flu Season 2025: Scientists Warn of Severe H3N2 Outbreak

New flu virus mutation could see ‘worst season in a decade’

Every winter, the flu strikes — but this year is proving to be different.

A seasonal flu virus underwent an unexpected mutation during the summer, sparking an early surge that began more than a month ahead of schedule. The altered strain appears to sidestep some of our existing immunity and, based on historical patterns, could lead to a harsher flu season.

The NHS has issued a “flu jab SOS” amid growing concern that this winter could be particularly difficult.

Leading flu experts say they wouldn’t be surprised if this turns out to be the worst flu season in a decade, though they emphasise that much remains uncertain.

“These dynamics are unusual; we haven’t seen a virus like this in a long time,” said Prof Nicola Lewis, head of the Francis Crick Institute’s World Influenza Center.
“It does concern me, absolutely. I’m not panicking, but I am worried,” she added.

Why Is This Happening?

Influenza viruses are constantly changing, which is why the seasonal flu vaccine must be updated every year. Scientists describe this process as a pattern of “shift and drift.”

Most of the time, the virus drifts — making small genetic tweaks. But every so often, it shifts, undergoing a significant mutation that alters how it behaves.

That’s what happened in June.

According to Prof Derek Smith, director of the University of Cambridge’s Center for Pathogen Evolution, seven genetic changes in a strain of H3N2 seasonal flu led to a “rapid rise” in reports of the modified virus.

“It almost certainly will sweep the world, so from that standpoint, it’s something that will come up quickly,” he told the BBC.

A Summer Mutation, a Winter Problem

Unusually, this shift occurred in the middle of summer — outside the usual flu season in the northern hemisphere.

By September, infections began to rise as children returned to school and temperatures started to fall.

Scientists are still studying the precise effects of these changes, but early evidence suggests the virus may now evade some of the protection that people have built up through years of vaccination and prior infection.

This may explain why the flu season has started so early in the UK and other parts of the world, including Japan. The virus is finding it easier to infect people — and spread quickly.

Flu Virus Mutates Early, Fueling Fears of a Severe Season

If the new flu strain can spread more easily, it may not have to wait for the typical winter conditions — when people spend more time indoors with the heating on and windows closed — to take off.

“We’re miles ahead,” said Prof Nicola Lewis, head of the Francis Crick Institute’s World Influenza Center. “I think it’s going to be a strong flu season.”

The latest mutation seems to have made the virus more transmissible. To put that in perspective, scientists use a figure known as the R number — the average number of people each infected person passes the virus to.

According to Prof Lewis, the R number for this year’s strain is estimated at 1.4, compared with the usual 1.2 for seasonal flu. In simple terms, if 100 people catch the flu in a normal year, they might infect 120 others. This time, that number could rise to 140.

Could This Be the Worst Flu Season in a Decade?

“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,” said Prof Christophe Fraser from the University of Oxford’s Pandemic Sciences Institute.

He added: “There are indicators that this could be worse than some of the flu seasons we’ve seen in the last 10 years.”

In a typical year, around one in five people catch the flu. But scientists warn the proportion could be higher this season.

Still, there’s a degree of uncertainty. Australia — which has just experienced its winter — recorded its worst flu season on record, even though it was not driven by the same H3N2 strain now circulating in Europe.

What is clear, experts say, is that the virus is spreading quickly among children — often the first to be exposed in schools.

“A 10-year-old’s immune system will look very different from their grandparents’, who’ve lived through six or more flu seasons,” Prof Lewis explained.

Over the coming weeks, researchers will be watching closely to see how the virus behaves as it begins to infect older and more vulnerable groups.

A More “Nasty” Strain Emerging

This year’s influenza appears to be more severe, particularly for older adults.

You might recognise some flu subtypes — H5N1 (avian flu) or H1N1 (swine flu) — both of which have caused global outbreaks in the past. The current concern centres on a mutated H3N2 strain, which scientists describe as more aggressive.

“H3 is always a hotter virus — a nastier virus — it’s more impactful on the population,” said Prof Lewis.

For most people, flu causes a sudden fever, muscle aches, and fatigue. But for the elderly and those with health conditions, it can be deadly.

Last year, flu claimed nearly 8,000 lives in the UK. During the 2022–2023 season, that number rose to almost 16,000. The NHS is now preparing for another challenging winter.

What Can Be Done?

The NHS has launched a “flu jab SOS” campaign, urging people to get vaccinated. More than 2.4 million appointments are available over the coming week.

“If you’ve been called by your GP, please get your flu vaccine as soon as possible,” said Prof Lewis. “This is absolutely the most important year to do it.”

However, this year’s vaccine is not a perfect match. Vaccine design decisions were made in February, before the new mutation appeared in June.

“Some protection is better than no protection,” said Prof Fraser. “But this year is likely to be one where the level of protection is lower than in years when the match is better. It’s not an ideal situation.”

Even so, the vaccine should still reduce illness severity and lower the risk of hospitalisation, particularly in older adults and people with chronic conditions.

“We can be confident that the vaccine will still help protect the most vulnerable from serious illness, regardless of which strains circulate this winter,” said Dr Mary Ramsay, head of immunisation at the UK Health Security Agency.

Global Efforts to Curb the Spread

Doctors have been reminded to prescribe antivirals early to patients at high risk, as this can reduce complications.

In Japan, where an early flu surge is also underway, schools have been temporarily closed to slow transmission — a short-term measure rather than a COVID-style lockdown.

As for what comes next, much remains uncertain.

“It might all go away by next week,” said Prof Lewis. “But I don’t think it will.”

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