Flu season comes around every year, but a new strain is leading many global health experts to worry that this round may be particularly severe. The strain—a version of the influenza A(H3N2) virus—first appeared in surveillance reports in June, 4 months after the 2025-2026 influenza vaccine formulation had already been determined, and has been associated with earlier waves of influenza outbreaks in Canada, Japan, and the UK.
After last year’s particularly severe flu season in the US, experts tracking global infection patterns are working to determine if the new H3N2 flu strain has the potential to lead to more severe illness or a less effective vaccine stateside.
“It’s too early to tell how severe the season is going to be,” said Shikha Garg, MD, MPH, medical officer of the influenza clinical epidemiology and treatment team at the US Centers for Disease Control and Prevention (CDC).
The H3N2 influenza virus is “quite common,” according to Jennifer Nuzzo, DrPH, professor of epidemiology and director of the Pandemic Center at Brown University School of Public Health. It’s a key cause of seasonal flu, alongside influenza A subtype H1N1 and influenza B viruses. But H3N2 has gained attention in recent months after developing a large number of mutations. This version is now known as H3N2 subclade K.
“It’s changed enough that it gets its own subclade designation,” Nuzzo said.
Alicia Budd, MPH, who leads the CDC’s domestic influenza surveillance team, noted that influenza viruses constantly change in small ways, and that this process, known as antigenic drift, has substantially affected the hemagglutinin surface protein in H3N2 subclade K. She confirmed approximately “10 additional mutations in that hemagglutinin gene that now make it different from the earlier, representative virus used to develop the vaccine.”
Although she explained that antigenic drift is typical and is why the flu vaccine makeup must be updated annually to keep up, the global rise in H3N2 infections, which often lead to more serious illness than other influenza viruses, has experts concerned.
“We’re seeing early warning signs from other countries,” said Anne Zink, MD, a senior fellow at the Yale School of Public Health and cofounder of PopHIVE.org, a platform that consolidates national health data from various sources, including the CDC.
Japan has reportedly experienced an unusually early flu season, with health officials declaring an influenza epidemic in early October. A similar start to the season has affected the UK, with the head of its National Health Service predicting in a press conference a “cruel winter, with flu cases being triple what they were this time last year.” And following a 29% increase in severe acute respiratory infections, including influenza, in the southern hemisphere, the Pan American Health Organization called for “strengthened preparedness to reduce the impact of respiratory viruses.”
According to Nuzzo, however, “We have not yet seen any indication that the US is having a flu season like Japan.”
Budd confirmed the timing of US flu activity is typical. “What we’re seeing right now is not anything remarkable—certainly not remarkably early.”
What she says her surveillance team is seeing is an increase in the proportion of H3N2 subclade K viruses in the US. “In the summer, about 40% of a very small number of H3 cases were subclade K, but now we’re observing about 90% of the H3 viruses belonging to that subclade.”
According to the CDC’s weekly FluView report for the week ending December 6, H3N2 viruses made up the majority of influenza A viruses reported by public health laboratories, around 86%, and subclade K accounted for about 89% of that group.
Dan Weinberger, PhD, professor of epidemiology at Yale School of Public Health, who analyzes data at PopHIVE with Zink, said that the H3N2 virus “does seem to be dominant currently” and is the primary driver of infections so far.
To Aaron Glatt, MD, the chief of infectious diseases at Mount Sinai South Nassau, the evolution of this particular flu strain is “unusual” but not unprecedented—even in terms of the flu shot’s viability.
“The subclade K strain has mutations that were not present in the strains of the 2025-2026 seasonal flu vaccine and thus might imply a mismatch that would potentially signify less benefit, but that remains conjecture,” he said. “Time will tell how important the mutations are in preventing optimal efficacy of this year’s vaccine.”
Considering that H3N2 is associated with more severe flu seasons than H1N1 or influenza B viruses, Garg said, even a slight mismatch between subclade K and the vaccine may be cause for concern.
“In the past, even without the drift, seasons where H3N2 viruses predominate have been linked to more influenza-associated hospitalizations and deaths in older adults, especially those 65 or older, and younger children compared to other age groups,” Garg said. “That’s something we’re looking out for.”
Budd also cautioned that H3N2 is disproportionately “hard on the oldest and youngest end of the spectrum” and leads to a greater disease burden than a typical flu season. So far this season, the CDC estimates 2.9 million illnesses, 30 000 hospitalizations, and 1 200 deaths from flu in the US.
Zink, a practicing emergency medicine physician, warned that flu surges can affect timely diagnosis and treatment and add strain on overtaxed health care systems.
And it’s not just about the subclade.
“Rather than one specific thing, it’s multifactorial,” Zink said. “A combination of what the strain is, how much it’s spreading, and the percentage of vaccine uptake determines how bad my year is and how bad my patient’s year is rather than any one specific thing.”
The US government shutdown that started in October and lasted a record 43 days limited access to actionable data, which may have affected vaccine uptake, Nuzzo pointed out.
Both Budd and Garg expressed that their teams at the CDC continued to monitor influenza activity throughout the shutdown and have since resumed publishing their data weekly, but to Nuzzo, the damage has been done.
“For 6 weeks, if you looked at the CDC’s website, it was like there was no flu,” she said, noting that the timing coincided with what would normally be the height of vaccine uptake.
Flu vaccinations have declined this season compared with the 2024-2025 season, according to the CDC. Data pulled from retail pharmacies and physician medical offices administering the flu shot show that vaccinations are down nearly 7% this year. About 44 million shots have been given so far this season—more than 3 million fewer than the number administered over the same time period in 2024.
“We’re potentially heading into a flu season that could be quite a doozy, and we know we’re starting it with fewer data and, more crucially, fewer vaccinations,” Nuzzo said.
During seasons with a drifted virus such as H3N2 subclade K, Garg anticipates that “individual- and population-level immunity might be low” and that vaccines may be less effective at preventing illness.
“That being said, the effect of antigenic drift on annual flu vaccine protection is not that easy to predict,” she said. “Even in past years with drift, we’ve seen substantial protection. For that reason, we’re still absolutely recommending vaccination.”
In fact, Glatt pointed to early real-world data from the UK, where the subclade is dominating, showing that the 2025-2026 vaccine has been 72% to 75% effective at preventing emergency department visits in children and 32% to 39% effective in adults. He considered such protection meaningful. “This is not really that different than what we see every year,” he said. Garg added that for each of the 3 influenza viruses, vaccine effectiveness in adults can range from 50% to 75%.
“I wish we had better flu vaccines that, if you got it, you’re not going to get the flu,” Nuzzo said. “That’s not the case. But in this vaccine, there will be H3N2, and that can provide some level of protection against severe illness and death, which is frankly the most important thing.”
To best prevent and mitigate influenza infections this season, Zink encourages clinicians to remind patients about more than just vaccines.
“A vaccine is like a splint for a broken leg. It can protect it, but you still need to take care,” she said. “Respiratory hygiene, staying away when you’re sick—those are all important, and H3N2 is even more reason to do it this year.”
Patients and clinicians can also consider antivirals, 4 of which are approved by the US Food and Drug Administration for influenza treatment.
“There’s no indication at this time that subclade K is resistant to the available antiviral medications, which are useful in seasons with a drifted virus as an adjunct to vaccination, especially for those who are at higher risk,” Zink said. For those patients, she said clinicians should recommend antiviral treatments, either as a preventive measure for those at heightened risk or as soon as possible after influenza infection is suspected or confirmed.
Zink admitted that predicting a flu season was akin to “reading the tea leaves.” Much is still unclear, she said. For now, the World Health Organization says there is no sign of increased disease severity based on the current epidemiological data.
Garg also acknowledged limitations to surveillance of the new strain—subclade K data aren’t often available at the individual patient level. “We’ll know if it’s influenza A or B or H3N2 or H1N1, but not by subclade,” she said. Still, she noted that the CDC is continuously monitoring the data to determine whether the amount or severity of cases this season differs from past seasons. “It’s still unfolding.”
