Flu activity is exploding across the United States this flu and cold season, in some cases reaching record highs, according to new Centers for Disease Control and Prevention data.

For the first time since 1997, when flu data was first tracked, 8% of visits to medical professionals were related to the flu or a flu-like illness the week ending Dec. 27, according to the CDC. Hospitalizations increased a whopping 48% week-over-week, amounting to 120,000 hospitalizations and 5,000 deaths so far this season.

A new mutation, called subclade K, continues to drive the spread, with some experts warning that it could supercharge the season. Here's what to know about the state of flu in the U.S. this week.

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How many flu cases were reported nationwide?

Flu cases have skyrocketed since December began, according to CDC data, and flu activity is now categorized as "high" nationwide.

CDC data through Dec. 27 showed that just under 33% (23,350) of the 70,757 flu tests administered that week were reported positive to the National Respiratory and Enteric Virus Surveillance System. This was up from just 9% during the first week of December.

In some regions, specifically Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming, that number was as high as over 45%. The CDC also noted that flu activity was "very high" in the Northeast the week ending Dec. 27, including in New York, Massachusetts and North Carolina.

Wastewater data maintained by WasteWaterSCAN, a Stanford University-led tool for tracking the spread of disease, also categorized flu activity as "high," marking a 146% increase in the median concentration between the first and last weeks of December, which means more of the virus has been detected in the wastewater of more communities nationwide.

As of Dec. 30, CDC projections found that flu infections are on the rise in 17 states, unchanged or not reported in seven states and declining or likely declining in 24.

Map of flu activity by state

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What is the new flu strain?

A new flu strain that has been dominating cold and flu season overseas has also taken over in the U.S.

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. 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 strains most commonly circulating. One of those, H3N2, has been present for decades, according to the CDC 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 shows 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 and research published in the medical journal Eurosurveillance.

The last available tracing data from the CDC found that 91% of the 600 U.S. flu cases that were tested between Dec. 22 and 28 were caused by H3N2. Additionally, 90.5% of the 389 H3N2 viruses sent for further testing during that time were subclade K.

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 best 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. Mask-wearing while sick or out and about during illness season can also help to prevent the spread.

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: Is the flu going around? See latest cases, map, symptoms of new strain