• Fast-spreading H3N2 subclade K emerged after this season’s vaccine was already finalized

ISTANBUL

A newly emerged strain of influenza A(H3N2) virus is spreading across multiple regions and prompting concern among global health experts, who warn it could contribute to a more severe-than-usual flu season, particularly among older adults and young children.

The strain, now classified as H32N subclade K, appeared after the 2025-2026 flu vaccine formulation had already been finalized, raising questions about vaccine match, disease severity, and preparedness as cases rise worldwide.

What is the virus?

The virus is a mutated form of influenza A(H3N2), one of the main drivers of seasonal flu, alongside H1N1 and influenza B.

“It’s changed enough that it gets its own subclade designation,” said Jennifer Nuzzo, professor of epidemiology and director of the Pandemic Center at Brown University School of Public Health.

The virus has undergone significant antigenic drift, accumulating approximately 10 new mutations in its hemagglutinin gene, the surface protein targeted by vaccines, according to the US Centers for Disease Control and Prevention (CDC).

“These mutations now make it different from the earlier, representative virus used to develop the vaccine,” said Alicia Budd, who leads the CDC’s domestic influenza surveillance team.

Where is it spreading?

The H3N2 subclade K strain has been linked to early and intense flu activity in several countries:

• Japan: Declared an influenza epidemic in early October, marking an unusually early start to the season
• UK: Health officials warned of a “cruel winter,” with flu cases expected to be three times higher than last year
• Canada: Reported early waves of influenza associated with the new strain
• Southern Hemisphere: A 29% rise in severe acute respiratory infections, including influenza, prompted the Pan American Health Organization (PAHO) to call for “strengthened preparedness to reduce the impact of respiratory viruses.”

In the US, health officials say overall flu timing remains typical, but the new strain is rapidly becoming dominant.

According to the CDC’s FluView report for the week ending Dec. 6, H3N2 viruses accounted for 86% of influenza A cases, and about 89% of those belonged to subclade K.

“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,” Budd said.

How severe could it be?

Experts caution that H3N2-dominated seasons have historically been linked to higher hospitalizations and deaths, especially among seniors and young children.

“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,” said Shikha Garg, a medical officer at the CDC. “That’s something we’re looking out for.”

So far this season, the CDC estimates 2.9 million illnesses, 30,000 hospitalizations, and 1,200 deaths from flu in the US. Separately, CBS News reported 4.6 million cases and 1,900 related deaths, including at least three children.

“This flu season is no joke,” said Dr. Amanda Kravitz, a pediatrician at Weill Cornell Medicine. “We are seeing more cases than we would expect for this time of year.”

Is the vaccine effective?

Because subclade K emerged after vaccine strains were selected, experts say a partial mismatch is possible, but not definitive.

“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,” said Dr. Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau.

“But that remains conjecture,” he added.

Early real-world data from the UK suggest the vaccine is still providing meaningful protection:

• 72%-75% effectiveness against emergency department visits in children
• 32%-39% effectiveness in adults

“This is not really that different than what we see every year,” Glatt said.

CDC officials continue to recommend vaccination.

“Even in past years with drift, we’ve seen substantial protection,” Garg said. “For that reason, we’re still absolutely recommending vaccination.”

Falling vaccination rates

Vaccination uptake has declined this season, compounding concerns.

Data by the CDC shows flu vaccinations are down nearly 7% compared with last year, with about 44 million doses administered, more than 3 million fewer than at the same point in the 2024-2025 season.

Nuzzo linked the drop in part to a 43-day US government shutdown that limited public access to flu data during peak vaccination weeks.

“For six weeks, if you looked at the CDC’s website, it was like there was no flu,” she said.

What are the symptoms?

Doctors say symptoms linked to the H3N2 strain can be intense and fast-onset, including:

• High fever (often 103–104°F)
• Severe body aches
• Cough
• Rapid spread within households and communities
• Vomiting in children

“Symptoms that are very intense, they come on really, really rapidly,” Kravitz said. “It’s very contagious.”

Parents are advised to seek medical care if high fevers persist beyond four to five days, especially in children.

What can be done?

Experts emphasize a layered approach:

• Vaccination, even if imperfect
• Early testing and antiviral treatment, such as Tamiflu or Xofluza
• Respiratory hygiene and staying home when sick

“A vaccine is like a splint for a broken leg,” said Anne Zink, MD, of Yale School of Public Health. “It can protect it, but you still need to take care.”

“There’s no indication at this time that subclade K is resistant to the available antiviral medications,” she added.

The World Health Organization (WHO) says there is no current sign of increased disease severity, but surveillance continues.

“We’ll know more as the season unfolds,” Garg said. “It’s still unfolding.”

Watching other threats

Experts are also monitoring avian influenza (bird flu) following a rare human case of H5N5 in Washington state.

“The risk to the general public from the H5N1 virus is still very low,” said Richard Webby, director of a WHO collaborating center. “But an H5 pandemic could easily make the COVID-19 pandemic look like a cake walk,” he added.

Separately, South Korea reported a new case of highly pathogenic avian influenza on Sunday at a duck farm in Naju, South Jeolla province, marking the 16th outbreak at a poultry farm this season.

Authorities have imposed special quarantine measures and stepped up monitoring of nearby facilities and vehicles, with inspections planned at more than 200 duck farms in the area starting Monday, Yonhap News reported.

Also, H5N1 bird flu was detected on Monday at a farm in Jincheon, North Chungcheong Province, bringing the total to 17 cases since the cold season began, according to the Central Disaster Management Headquarters (CDMH).

Health officials urge vigilance, but not panic.

With a fast-evolving virus, declining vaccination rates, and early global warning signs, experts say preparedness, surveillance, and early care will shape how severe this flu season ultimately becomes.

*Quotes from medical professionals mentioned appeared in reports by CBS News and the medical journal JAMA Network Open.