Flu Is Beginning to Hit the U.S. Hard Driven by Influenza A Variant Subclade K

Flu is currently spreading throughout the United States, and is likely to accelerate as people begin to gather for the holidays and children are on school break.

The most recent data from the Centers for Disease Control and Prevention (CDC) show that influenza infections, hospitalizations, and deaths are all on the rise.

This year, flu has already caused:

4.6 million illnesses
49,000 hospitalizations
1,900 deaths, including at least three children

“While the severity of any flu season is notoriously difficult to predict, it appears that influenza activity is tracking similarly to the 2024–2025 flu season, which was severe and saw a record number of pediatric deaths” — nearly 300 — says Robert H. Hopkins, MD, medical director of the National Foundation for Infectious Diseases.

In its latest report, the CDC describes overall severity indicators for flu (such as hospitalizations and deaths) as low, but influenza activity is expected to continue its miserable march forward for weeks to come.

Although flu activity typically peaks between December and February, the virus season can extend as late as May, the CDC says.

At Least 14 States Have High or Very High Flu Activity

The most recent tracking data show that flu-like illness is hitting high or very high levels in 14 states — Colorado, Connecticut, Georgia, Idaho, Louisiana, Maryland, Massachusetts, Michigan, New Jersey, New Mexico, New York, North Carolina, Rhode Island, and South Carolina — as well as Washington, DC, and Puerto Rico.

While flu activity remains low to moderate in other parts of the United States, residents should not let their guard down, says Dean Winslow, MD, an infectious disease specialist and professor of medicine and pediatrics at Stanford Health Care in California.

“Differences may exist now, but my sense is they are going to largely disappear by mid- to late-January,” he says.

Subclade K Is Fueling the Spread

The surge in illnesses relatively early in the flu season has been concerning to many infectious disease specialists, as a new version of influenza A (H3N2) called subclade K appears to be the driving force.

In the most recent flu samples tested, scientists found about 90 percent were H3N2, which is a common subtype of flu virus — and nearly 9 in 10 of those samples examined for genetic characteristics turned out to be a new subvariant known as subclade K.

Data from the past few weeks suggested that subclade K has been propelling rising flu cases in Japan, Canada, and the United Kingdom. On Thursday, U.K. health authorities said levels were beginning to stabilize, though they warned the season could still be unpredictable.

H3N2 subclade K has seven unexpected mutations that increase its ability to break through immune defenses, including those provided by vaccination.

“This looks to be a particularly bad flu season because the current predominant strain, H3N2, has had a significant accumulation of mutations when compared to prior years,” says Gabriela Andujar-Vazquez, MD, an infectious disease doctor and epidemiologist at Dartmouth Health in Lebanon, New Hampshire.

“This is what drives seasonal influenza epidemics” — when the disease spreads rapidly to many people — “and why we need to update influenza vaccines every year to keep up with the virus mutations,” Dr. Andujar-Vasquez says.

Flu Shot Offers Significant Protection

Annual flu vaccine formulas for the Northern Hemisphere are updated based on variants that circulated during the flu season that just passed in the Southern Hemisphere.

Because subclade K took off at the tail end of the Southern Hemisphere flu season, vaccine makers in the Northern Hemisphere did not target it, as reported by the Center for Infectious Disease Research and Policy Research at the University of Minnesota.

Even though this year’s flu vaccine is somewhat of a mismatch with subclade K, infectious disease experts stress that it should provide significant protection against serious illness and hospitalization because it contains components to protect against the H3N2 strain in general.

“Vaccination is the best tool for preventing severe flu,” says Dr. Hopkins. “Even though the H3N2 strain has drifted from the strain selected for current flu vaccines, these vaccines should provide protection against severe disease.”

So far this year’s flu vaccine effectiveness (its ability to keep people out of the ER or hospital) seems to be in the typical range, according to early analysis, meaning:

72 to 75 percent effectiveness for children and adolescents
32 to 39 percent effectiveness for adults

What Are Flu Symptoms of Subclade K and Who’s Most at Risk?

“Flu is not typically subtle or mild like the common cold,” says Andujar-Vazquez. “People feel pretty sick for the first couple of days.”

Flu symptoms this season appear to be the usual suspects, she adds.

Common symptoms of the flu include:

Fever (but not everyone with the flu develops fever) or feeling feverish
Cough
Sore throat
Runny or stuffy nose
Muscles or body aches
Headaches
Fatigue
Vomiting or diarrhea (more common in children)

People at the highest risk of serious flu complications include:

Adults 65 and over
Children younger than 2
People with weakened immune systems and chronic illnesses, such as asthma, diabetes, and heart and lung disease

“The number of hospitalizations and deaths each year really highlights how serious the flu can be,” says Dr. Winslow. “This isn’t just a disease that kills older people — it can be a very serious illness for young people as well.”

Tips to Avoid Getting Flu or Another Virus This Winter

Public health authorities encourage commonsense measures to protect against the spread of flu and other respiratory illnesses, including the common cold, COVID-19, and RSV:

Avoid close contact with people who are sick.
Limit contact as much as possible if you yourself are sick.
Wash your hands often with soap and water.
Consider wearing a mask around other people.
Cover coughs and sneezes.

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