A New Flu Strain Is Dominating This Winter—Here's What You Need to Know

A new influenza A(H3N2) virus variant called subclade K has become the dominant flu strain circulating in the United States, accounting for approximately 90% of genetically characterized H3N2 samples since late September. This emergence is raising concerns among health experts about vaccine effectiveness and the potential for more severe illness, particularly in children who are experiencing alarming rates of serious complications.

What Makes Subclade K Different From Other Flu Strains?

Subclade K represents a significant shift in the flu landscape. The virus has genetic differences from other circulating influenza viruses, including the A(H3N2) component that was selected for the 2025-2026 seasonal flu vaccine. This mismatch between the vaccine strain and the dominant circulating virus raises questions about how well current vaccines will protect people this season.

The CDC first identified subclade K in the United States in August 2025, and its rapid rise to dominance has been striking. As of December 19, 2025, approximately 90% of influenza A(H3N2) viruses that the CDC genetically characterized from U.S. samples collected since September 28 were identified as subclade K. This represents a dramatic shift in just a few months.

Why Are Doctors Concerned About Severe Complications?

Medical experts are sounding the alarm about serious flu complications emerging this season, particularly in children. Pediatric infectious disease specialists report that the flu's ability to damage the body's natural defenses makes it especially dangerous, even though many people dismiss it as a routine seasonal illness.

The complications being documented include:

• Pneumonia: One-third of admitted flu patients develop pneumonia, according to pediatric specialists
• Encephalitis: Inflammation of the brain that can cause serious neurological damage
• Myocarditis: Inflammation of the heart muscle that can affect cardiac function
• Viral myositis: A muscle condition most often seen in children during or shortly after flu infection, causing temporary but alarming symptoms like sudden inability to walk

What makes this season particularly worrying is the extremely low vaccination rates. Since the end of the COVID-19 pandemic, the number of people becoming sick with influenza has drastically increased each respiratory illness season, largely due to vaccination hesitancy.

How Severe Will This Flu Season Be?

The CDC expects the 2025-2026 influenza season will most likely be classified as moderate across all ages, though the agency has low confidence in this assessment. The uncertainty stems from questions about how the new subclade K virus will impact population immunity and vaccine effectiveness against circulating strains.

As of December 16, 2025, influenza infections were growing or likely growing in 47 states, with the timing of this increase similar to several past seasons. Influenza A(H3N2) viruses remain the most frequently reported influenza viruses so far this season, with subclade K being the predominant variant not just in the United States but globally, including in the United Kingdom and Canada.

Current respiratory illness activity is at moderate levels nationally, with seasonal influenza activity remaining elevated. Most areas of the country are reporting stable or decreasing trends, though activity continues to increase in the Pacific Northwest.

What Can People Do to Protect Themselves?

Despite concerns about vaccine strain matching, health experts emphasize that vaccination remains one of the strongest tools for maintaining health and preventing serious illness. "Vaccines remain one of the strongest tools for maintaining health and preserving natural immunity over a lifetime," explains Dr. Doug Kasper, an infectious disease specialist with OSF HealthCare.

As of December 6, 2025, only 15.3% of adults 18 years or older had received the updated 2025-2026 COVID-19 vaccine, which is lower than the same time last year when 19.2% had been vaccinated. Vaccination rates for flu appear similarly concerning given the warnings from medical professionals.

Beyond vaccination, experts recommend standard preventive measures during the respiratory illness season. These include practicing good hygiene, wearing masks if you have symptoms, staying home when sick, and maintaining distance from others who are ill. Healthcare facilities are also implementing precautions, including offering complimentary masks to visitors and limiting visitor numbers to protect vulnerable patients.

The emergence of subclade K underscores the unpredictable nature of influenza and why annual vaccination, even when not perfectly matched to circulating strains, remains important for reducing severe illness and death. As flu activity continues through the winter months, staying informed about current trends in your community and taking preventive action remain the best strategies for protecting yourself and others.