A new flu strain that has been dominating cold and flu season overseas is on the rise in the United States.

The mutated strain, known as subclade K, originates from the most common flu-causing virus, influenza A, and one of its most common variations, H3N2. This could mean a particularly brutal flu season, especially with the current vaccine being a mismatch to the rapidly-spreading variant.

There's no need to panic yet, though - as we get deeper into flu season and more data becomes available, early evidence is emerging that the vaccine still provides good protection against serious illness from subclade K.

As family gatherings and holiday celebrations approach, here's what to know about the new flu strain.

What is the new flu strain?

Subclade K (previously called A(H3N2) virus subclade J.2.4.1) is a mutation of influenza A H3N2. Influenza A is typically the virus associated with the term "flu" and has evolved into many different strains over time.

Each year, the seasonal flu shot is formulated to target the most commonly circulating strains. One of those, H3N2, has been present for decades, according to the Centers for Disease Control and Prevention and the Gavi Vaccine Alliance, but its most recent mutation is different enough that the vaccine made to target H3N2 for the 2025-2026 season is not as effective against it.

Because it is structurally different, subclade K may also be better at dodging immune systems that have already built up protections against more familiar strains. This year's vaccine isn't totally useless against subclade K, however.

Preliminary data has shown that the vaccine is still providing protection and rates of hospitalization and serious illnesses are comparable to last year in Europe, according to the Pan American Health Organization (PAHO) and research published in the medical journal Eurosurveillance.

Subclade K spread quickly in Europe, the UK, Japan and several East Asian countries early this flu season, before being detected in the United States and Canada. It was first seen in the U.S. as early as August, according to the CDC.

Health officials in these countries have not reported a significant change in the severity of flu cases as of December, though past trends show that older adults could be more heavily impacted, according to PAHO.

What's going on with flu subclade K in the U.S.?

Subclade K has become the most-detected H3N2 virus in Europe and Asia, according to PAHO, and testing in the U.S. has turned up an increasing number of cases, as well.

The last available tracing data from the CDC found that 86% of the 547 flu cases that were tested between Dec. 1 and 7 were caused by H3N2. Additionally, 89% of the 163 H3N2 viruses sent for further testing between Sept. 26 and Dec. 7 were subclade K.

What's going on with flu in the U.S.?

In general, flu activity is up and rising, with the CDC saying in its Dec. 11 weekly update that "flu season is just starting."

Influenza A concentrations saw a dramatic increase of 260% since October, according to WasteWaterSCAN data through Dec. 12. Even so, flu activity is still considered "low" nationally, according to both WasteWaterSCAN and CDC data.

CDC data through Dec. 6 showed that 8.05% (4,779) of the 59,364 flu tests administered that week were reported positive to the National Respiratory and Enteric Virus Surveillance System. In some regions, specifically in Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming, that number was as high as over 20%.

Some regions of the United States have notably higher flu activity than others. The Northeast and South are now officially considered to be in flu season and have higher concentrations than the rest of the country.

As of Dec. 9, CDC projections found that flu infections are on the rise in 43 states, and unchanged or not reported in seven states. No states reported a decline.

What are the latest flu symptoms?

Symptoms associated with H3N2 are similar to those of other influenza viruses and include:

• Fever

• Cough

• Runny nose

• Fatigue

• Muscle aches and chills

How to protect yourself from the flu

You may be tired of hearing it, but the old cliche is true: Washing your hands well − and often − is one of the most effective ways to keep contagious illnesses at bay, according to the CDC. This applies to all the ailments above and others that often circulate during the wintertime, like respiratory syncytial virus, or RSV.

Health agencies recommend washing your hands with hot, soapy water for at least 20 seconds. It's also important to wash them before and after eating, before and after handling food, after using the bathroom and after being around sick people or touching surfaces that could be contaminated.

The most effective way to prevent serious illness from flu is vaccination, according to the World Health Organization, CDC and other leading medical institutions. Everyone 6 months and older is eligible for the flu vaccine unless they have a specified medical condition, such as an allergy to its ingredients or a previous severe reaction to the shot.

This article originally appeared on USA TODAY: A new flu strain is quickly spreading. How to stay healthy.