Across parts of North America and Europe, the holiday season is usually a time for reunions, travel and crowded celebrations. However, this winter, a fast-moving flu strain is making its presence felt
just as Christmas approaches, raising concerns that the virus could disrupt plans and put extra strain on health systems.
The variant, a subgroup of the common Influenza A H3N2 virus called subclade K, has been reported in major cities and according to officials, it as a seasonal flu virus that’s spreading more efficiently than usual, earning the moniker “super flu”.
As per the World Health Organisation, the flu has been described as a seasonal respiratory infection, with symptoms ranging from fever and cough to body aches, sore throat and congestion.
What Is Subclade K?
According to BBC, subclade K is a genetically distinct offshoot of the Influenza A (H3N2) virus, identified through routine global surveillance that tracks how flu viruses evolve over time.
Scientists group influenza viruses into clades and subclades based on small genetic changes, much like drawing branches on a family tree. When researchers analysing virus samples noticed a cluster of H3N2 viruses sharing the same mutations—and behaving differently enough from earlier strains—they classified it as a new subclade and labelled it “K” for ease of reference.
The BBC report adds that the naming does not mean the virus is entirely new; rather, it signals that this version has drifted genetically from previous H3N2 strains, which can affect how easily it spreads and how well existing immunity recognises it.
What Are The Symptoms To Look Out For?
High fever, chills
Persistent cough and sore throat
Congestion or runny nose
Body aches and headaches
Extreme fatigue
Some cases include nausea or vomiting (especially in children)
In severe cases: difficulty breathing, chest pain, confusion or persistent high fever, which require prompt medical attention.
Why Is The Spread Faster Compared To Traditional Flu?
According to the Center for Disease Control and Prevention (CDC), viruses like influenza constantly mutate, and subclade K is a slightly drifted version of the H3N2 strain. This means it has genetic differences from the strains used to formulate the 2025-26 seasonal flu vaccine, potentially influencing how well immune systems recognise it.
CDC reported that most H3N2 viruses identified in the United States this season belong to subclade K.
Looking to allay concerns, health experts clarified that the mutation doesn’t create a completely new disease. It still behaves like influenza but may spread more easily because fewer people have immunity from past exposure or vaccination.
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, pushing up the flu numbers this season.
However, in his conversation with BBC, Lawrence Young, professor of molecular oncology at the University of Warwick, warned against “exaggeration” and said: “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.”
How Does The Virus Spread?
Like other flu viruses, the new strain spreads mainly through respiratory droplets when infected people cough, sneeze or talk; close physical contact in crowded or indoor settings; contaminated surfaces touched before touching the mouth, nose, and eyes. This is also the reason why the virus travels faster in winters when indoor gatherings are common.
People most vulnerable to severe illness include very young children, particularly those below the age of two, seniors, expectant mothers, and individuals living with long-term health issues such as asthma, diabetes, heart or neurological conditions, or compromised immune systems.
In the United States, the impact has been most visible in New York City. According to Fox40, health officials logged roughly 14,000 flu infections in the first week of December alone — an increase of more than four times compared to the same period last year.
Explaining the sharp rise, Dr Ulysses Wu, chief epidemiologist at Hartford Hospital, told NewsNation that New York’s role as a major travel hub makes it especially vulnerable. He noted that the city’s dense population, combined with heavy international and domestic movement, helps fuel faster transmission.
The surge is not limited to one region. Speaking at a public briefing on December 16, infectious disease expert Dr Andrew Pekosz said influenza activity is accelerating across several states. He pointed to Colorado, Louisiana and New York as areas seeing particularly steep increases, adding that in states such as Maryland, the subclade K variant appears to have become the dominant strain.
In the UK, pressure on hospitals remains elevated. BBC reported that an average of 3,140 patients were admitted with flu in England last week — an 18 percent rise from the previous week, although the pace of growth has begun to ease. The UK Health Security Agency has cautioned that it is still “too early to say if flu has peaked.”
Canada is also reporting a sharp upswing. Data from Health Canada, cited by Globe News, indicates that the H3N2 strain is spreading rapidly, with a significant number of cases requiring hospital treatment. Children and teenagers under the age of 19 are among the most affected groups.
The spread of the same variant has been observed beyond North America and Europe. Dr Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, told TODAY.com that Japan has already seen a major surge linked to this strain, prompting authorities there to declare an influenza epidemic.
Can You Take A Vaccine?
While getting vaccinated against the flu is advised, the current flu shot may be less perfectly matched with this variant than usual.
So, What Can You Do To Stay Safe?
Frequent handwashing
Staying home when symptomatic
Using masks indoors during high transmission
Avoiding close contact with infected individuals
Good ventilation in shared spaces
