US issued urgent warning about symptoms as mutated virus strain that causes ‘more severe disease’ sweeps nation

As a mutated strain of influenza sweeps the globe, medics have shared the key symptoms to look out for as flu season begins.

The new version of influenza A H3N2 emerged earlier this year and has caused widespread hospitalizations in countries such as Japan, Australia and the UK, with thousands of people affected by the unusually severe illness.

While flu isn’t uncommon in the US, the new variant is currently proving to be more infectious and severe than other strains, largely due to having picked up a number of mutations which mean it is less likely to respond to this year’s flu vaccines which were designed to target a different strain of the virus.

The mutated H3N2 strain, called 'subclade K', has spread rapidly across the globe, with Japan even declaring it an epidemic after experiencing an unusually early and harsh flu season this year.

Just under 200,000 cases were reported in the country between November 17 and November 23, with multiple schools being closed in order to try and get a handle on the spread of the illness.

Unlike during the pandemic, these were not full-scale lockdowns, but rather a short-term solution to help try and slow the spread of the infection.

The virus could be particularly severe for children (MoMo Productions / Getty Images)

The UK also reported that it expects to see the ‘worst flu season’ in over a decade, with Canada also reporting high numbers of infection.

So far, H3N2 subclade K has been detected in more than half of all U.S. states, and experts are warning people that it could prove particularly nasty for the elderly and children.

“Knowing that there’s a new mutated strain out there and H3N2 generally causes more severe disease is concerning,” Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, told TODAY.com.

Peak flu season typically hits between December and February, but has been known to extend into May, according to the US Centers for Disease Control and Prevention.

In the US, there are four types of Influenza, A, B, C and D.

The most common types are Influenza A (including strains such as H1N1 and H3N2) and Influenza B, and this year’s annual flu shot has been designed to target these variants. However, H3N2 has mutated since the vaccine was engineered to produce a far more aggressive strain that isn’t targeted by the current jabs.

The subclade K H3N2 strain currently has seven new mutations.

We've just entered peak flu season (Getty Stock Image)

What are the symptoms?

The symptoms of the mutated H3N2 flu strain follow the usual pattern of seasonal flu, and include:

• Fever

• Chills

• Body aches

• Headache

• Extreme fatigue

• Congestion or runny nose

• Coughing

The symptoms can come on suddenly, sometimes overnight and can look similar to those caused by COVID-19 and other viruses.

Flu is usually self limiting, but can prove fatal if severe cases are left untreated (Jelena Lalic / Getty Images)

How is flu treated?

Influenza is generally a self-limiting illness that can be managed at home with plenty of rest and fluids and will typically resolve itself in five to seven days.

However, if your symptoms are severe or you have a high fever, signs of dehydration, or trouble breathing, it is important to seek advice from a medical professional.

"The flu is not just a cold. It can be quite severe," Hopkins warned.

Approximately 28,000 died in the US from flu in the 2023-2024 season, with around 470,000 people being admitted to hospital.