A mutated strain of the influenza A H3N2 virus is driving a sharp rise in flu cases across the United States, worrying doctors as winter approaches. Health experts say this new variant, known as subclade K, has already caused early and intense flu seasons in parts of Asia, Australia and Europe, and the US may now be seeing the start of a similar pattern.
According to the US Centers for Disease Control and Prevention (CDC), flu activity crossed a key threshold in early December. For the week ending December 6, 3.2 per cent of doctor visits nationwide were for flu-like symptoms such as fever accompanied by a cough or sore throat. This figure is higher than the national baseline used by experts to mark the start of flu season.
“This is the signal that flu season is officially underway,” said Dr Caitlin Rivers, who directs the Center for Outbreak Response Innovation at the Johns Hopkins Bloomberg School of Public Health.
Flu season begins earlier than expected
Flu season is usually tracked in two ways. One is by the calendar, with week 40, or early October, often cited as the start. The other, and more meaningful measure, is actual flu activity. Experts say the season truly begins once doctor visits for flu-like illness cross the 3.1 per cent mark.
“This threshold tells us that flu is spreading widely in the community,” Rivers explained. The CDC’s latest update shows that at least 14 public health jurisdictions are already reporting moderate to high flu activity. Much of this activity is concentrated in the Northeast. New York City is experiencing very high levels, while New York and New Jersey are classified as high.
Connecticut, Massachusetts and Rhode Island are seeing moderate activity. Flu cases are also rising sharply in states such as Louisiana and Colorado, with other increases reported in Georgia, South Carolina, Texas, Puerto Rico and Idaho. This week also marked a grim milestone: the first reported flu-related death of a child in the US this season.
Doctors warn of a potentially rough season
Some states are already seeing worrying trends. In New York, flu cases began rising nearly two weeks earlier than usual, according to Dr James McDonald, commissioner of the New York State Department of Health.
“It’s here, and it announced itself quite loudly,” McDonald said. Hospitalisations in the state have doubled each week for the past two weeks. When plotted on a graph, the rise forms a sharp J-shaped curve, indicating a rapid increase in severe cases.
“That kind of curve is not common,” McDonald said. “It means more people are getting sick faster and ending up in hospitals sooner than we usually see.”
The state health department has issued alerts to hospitals, urging them to closely monitor flu samples, identify subtypes and strengthen infection control measures.
What is subclade K?
Laboratory data from across the US shows that most flu viruses analysed this season belong to subclade K, a mutated version of the H3N2 strain of influenza A. H3N2 seasons are historically associated with more severe illness, particularly among older adults.
“Seasons dominated by influenza A(H3) often result in more people seeking medical care and higher rates of hospitalisation,” a New York health alert noted.
Other countries are seeing similar patterns. Japan, China, the UK and Canada have all reported busy flu seasons. Australia, where subclade K was first identified, recorded nearly half a million confirmed flu cases, breaking records set just a year earlier.
Public health experts often look to Australia’s flu season, which runs from April to October, for clues about what might happen in the Northern Hemisphere. However, flu seasons remain unpredictable.
“There’s an old saying in public health: if you’ve seen one flu season, you’ve seen one flu season,” said Dr Jennifer Nuzzo, director of the Pandemic Center at Brown University.
Nuzzo noted that the US had a particularly bad flu season last year, and it’s uncommon to have two severe seasons back-to-back. Still, she cautioned that the presence of a new variant could change expected patterns.
Are flu vaccines working?
This year’s flu shots were finalised before subclade K was identified, so the variant is not directly included in the vaccine. However, the shots do contain closely related strains, and early data suggests they still offer meaningful protection.
A study by the UK Health Security Agency found that while antibodies from this year’s vaccines were less effective at neutralising subclade K, the shots still reduced emergency room visits and hospitalisations by nearly 75 per cent in children. Among adults, including those over 65, vaccines lowered the risk of severe illness by about 30 per cent to 40per cent.
Whether the US will see similar results remains uncertain. Most vaccines in the UK are cell-based, while the US relies largely on egg-based vaccines, which may not perform as well against some variants.
“I hope we see similar protection, but I’m a little worried,” Rivers said. Despite these concerns, experts agree that vaccination remains the best defence. “Any protection is better than none,” said Dr Alex Greninger, head of infectious disease diagnostics at the University of Washington.
It takes about a week after vaccination for immunity to fully develop, so doctors urge people not to delay. “If you get vaccinated now, your immune system will be ready by Christmas,” Greninger said.
Children at higher risk
Vaccination is especially important for children, who have had fewer past exposures to flu viruses and therefore less natural immunity. Yet CDC data shows that only 38% of children in the US have received a flu shot so far this season, lower than in previous years.
Last year, the US recorded at least 280 flu-related child deaths, the highest number in a non-pandemic year since tracking began in 2004. In cases where vaccination status was known, nearly 90% of the children who died were unvaccinated.
How to protect yourself this winter
Flu shots do not completely prevent infection or transmission, but they significantly reduce the risk of severe illness, hospitalisation and death. Because vaccines are not perfect, experts recommend layering protections.
As flu cases rise, Rivers says she wears a mask in crowded indoor spaces like airports and avoids high-risk environments. Improving ventilation also helps. Opening windows, using air purifiers and bringing in fresh air can reduce the spread of viruses, especially during gatherings.
If flu symptoms strike suddenly including high fever, cough, sore throat, body aches and chills testing is important. Antiviral medications can reduce the severity and length of illness if started within the first 48 hours.
Disclaimer: The information in this article is for educational and informational purposes only and is not intended as medical advice. While the author has incorporated expert medical guidance while producing the story and ensured full authentic information is provided to the reader, you should always seek the advice of a qualified healthcare provider regarding a medical condition or treatment. Never disregard professional medical advice or delay in seeking it because of something you have read here.