After a particularly rough December with record-high health care visits, flu cases are on the decline so far in January.

Centers for Disease Control and Prevention data previously found that 8% of visits to medical professionals were related to the flu or a flu-like illness the week ending Dec. 27, the highest since tracking began in 1997. While these numbers were on a downward trajectory the week ending Jan. 3, the season has already proven especially deadly for children, with eight new pediatric deaths being reported. In total, 17 known children have died from influenza-associated disease this season.

"Elevated influenza activity is expected to continue for several more weeks," said the CDC, which has recorded 180,000 hospitalizations and 7,400 flu 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 United States this week.

How many flu cases were reported nationwide?

Flu cases skyrocketed through December but have taken a downward turn in January. Flu activity is categorized as "moderate" nationwide, according to CDC tracking.

CDC data through Jan. 3 showed that just under 25% (25,357) of the 102,518 flu tests administered that week were reported positive to the National Respiratory and Enteric Virus Surveillance System. This was down from approximately 32% during the previous week ending Dec. 27.

In some regions, specifically Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming, that number was as high as 40%. The CDC also noted that flu activity was high to very high in the Northeast the week ending Jan. 3, including in New York, Maine and Massachusetts.

Wastewater data maintained by WasteWaterSCAN, a Stanford University-led tool for tracking the spread of disease, categorized flu activity as "high" but marked a 37% decrease in the median concentration between a peak on Dec. 26 and Jan. 7, which means less of the virus has been detected in the wastewater of more communities nationwide.

As of Jan. 6, CDC projections found that flu infections were on the rise in one state, unchanged or not reported in three states and declining or likely declining in 44. While the overall trend was declining, the weekly percentage of emergency department visits related to flu remained high.

Map of flu activity by state

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.

Flu cases spike with subclade K: What you need to know.

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 92.3% of the 885 U.S. influenza type A cases that were tested between Dec. 28 and Jan. 3 were caused by H3N2. Additionally, 91.5% of the 436 total H3N2 viruses sent for further testing between Sept. 28 and Jan. 3 were subclade K.

What are the latest flu symptoms?

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

• Muscle aches and chills

• Fever

• Cough

• Runny nose

• Fatigue

Should you get the shot? 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 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.

Vaccine schedule change: RFK Jr. says fewer flu shots for kids may be 'better.' What experts say.

Until recently, everyone 6 months and older was eligible for the flu vaccine unless they had a specified medical condition, such as an allergy to its ingredients or a previous severe reaction to the shot.

The CDC made significant changes to the childhood vaccine schedule on Jan. 6 under the guidance of Health and Human Services Secretary Robert F. Kennedy Jr. The agency removed universal recommendations for childhood vaccines that protect against RSV, meningococcal disease, COVID-19 and the flu, instead suggesting only children at "high risk" of serious illness receive them or healthy children only get them after consulting with a doctor.

People over the age of 18 can still walk into their local pharmacy and get a flu vaccine at their discretion. Parents looking to get their children vaccinated should check with their health care provider and local pharmacy to ensure all requirements are met.

This article originally appeared on USA TODAY: See where flu season is hitting America the hardest in 2026