‘Super flu’ spreads across US, UK and Canada: What is H3N2 subclade K strain and why it is spreading faster

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As the holiday season approaches, health authorities across the United States, the United Kingdom and Canada are dealing with a fast-spreading strain of influenza that has disrupted normal winter patterns. The virus, a mutated form of influenza A H3N2 known as subclade K, has emerged as the dominant flu strain this year and is driving sharp spikes in infections and hospitalisations.

Though widely labelled the “super flu,” global health agencies say it is still a seasonal influenza virus. What is worrying doctors is how quickly it is spreading and how effectively it appears to be bypassing existing immunity from vaccines and past infections.

The World Health Organisation has described it as a seasonal respiratory infection, with symptoms ranging from fever and cough to body aches, sore throat and congestion.

The so-called super flu is a mutated version of influenza A H3N2 that scientists have identified as subclade K. It emerged earlier this year and has since spread rapidly across several regions.

Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, explained the science behind the name. “The H and N refer to haemagglutinin and neuraminidase, two proteins on the surface of the virus,” he told The Telegraph. “The numbers simply classify which versions of those proteins are present. It also helps us categorise strains. H3N2 just happens to be the variant dominating this year.”

Medical experts say influenza viruses constantly evolve, allowing them to evade immune defences. Professor Ed Hutchinson from the Glasgow Centre for Virus Research told the publication, “We have had it and already recovered well, but a lot of people are getting infected.”

Doctors say immunity tends to be weaker during winter months, which may be contributing to the unusually fast spread.

Health experts warn that this flu season could be harsher than usual. Dr Scott Roberts, assistant professor of infectious diseases at Yale School of Medicine, told The New York Times that the subclade K strain may spread more widely than previous variants.

He said it may be better at avoiding immunity from vaccines and prior infections, leading to higher overall case numbers. Referring to last winter, Dr Roberts added, “Last year’s flu season was the roughest in nearly a decade.”

“But a lot of us are concerned that with this new, slightly more mutated than usual subclade K strain, we may see that,” he warned.

However, not all experts agree with the alarmist label. Lawrence Young, professor of molecular oncology at the University of Warwick, cautioned against exaggeration. “It’s just a flu variant that is clearly a little bit more infectious than normal, but it’s misleading and a bit frightening to call it super flu,” he told the BBC.

Despite being a new variant, the symptoms closely resemble seasonal flu. These include a sudden high temperature, weakness, headaches, muscle aches, a dry cough, sore throat, runny or blocked nose, loss of appetite, stomach pain and difficulty sleeping.

Doctors say what sets this strain apart is how quickly symptoms appear. According to The New York Times, patients experience fever, chills, congestion and full body aches more abruptly than with Covid, which often begins with milder symptoms.

Doctors have flagged serious warning signs. Dr Aslam told The Telegraph that “difficulty breathing, chest pain, confusion, persistent high fever, severe dehydration, or symptoms that suddenly worsen” should prompt immediate medical attention.

Those at higher risk include children under five, especially under two, older adults, pregnant women and people with chronic conditions such as asthma, diabetes, heart disease, weakened immune systems or neurological disorders.

There is no single cure for the virus. The UK’s National Health Service has advised basic measures such as resting, drinking fluids, using paracetamol or ibuprofen to reduce fever, and avoiding work or school until fully recovered.

Dr Clarke told The Telegraph there was “no magic bullet”. He said preventive basics still matter, including handwashing, avoiding crowded indoor spaces and staying home when sick.

Doctors also say lessons from the pandemic remain relevant, including mask use in crowded settings and working from home when possible.

Vaccination remains important, particularly for children, who catch and spread flu more easily.

The spread has been particularly sharp in major population centres. In the United States, Fox40 reported that New York City has been hit hardest. During the first week of December, officials recorded about 14,000 flu cases, a 460 percent jump from the same period last year.

Dr Ulysses Wu, chief epidemiologist at Hartford Hospital, told NewsNation, “We see the sudden spike in New York because of their more dense population. They’re at the centre for international as well as domestic travel.”

Dr Andrew Pekosz, an infectious disease specialist, said the virus is spreading widely across the country. During a public briefing on December 16, he said, “Colorado, Louisiana and New York are that are experiencing really fast increases in influenza. In places like Maryland, the subclade K virus seems to be the dominant one.”

In the UK, hospital admissions remain high. BBC reported that an average of 3,140 patients were hospitalised with flu in England last week. Cases were up 18 percent from the previous week, though the rate of increase has slowed.

The UK Health Security Agency has said it is still “too early to say if flu has peaked”.

Canada is also seeing a rapid rise. Health Canada data cited by Globe News shows the H3N2 strain is spreading quickly, with many cases requiring hospital care. Children and teenagers under 19 are among those most affected.

Dr Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, told TODAY.com that the same variant has caused a surge in Japan, where authorities have already declared an influenza epidemic.