New 'super flu' H3N2 subclade K drives early winter surge across the U.S.

A new and potentially dangerous influenza A strain, H3N2 subclade K, is driving an earlier-than-usual surge in flu activity across the United States, federal data show. Health officials warn the subclade’s spread may amplify illness this winter as people travel for the holidays and gather with family and friends. While the full impact remains to be measured, public health experts say the lineage could be contributing to higher case counts ahead of the typical peak season.

CDC data updated in early December show the season has already produced substantial illness. About 3 million Americans have tested positive for influenza this season, and roughly 1,200 deaths have been reported so far. New York City appears to be at the center of higher activity, with the metropolitan area and several nearby states recording elevated levels. The broader tri-state region and states such as Colorado and Louisiana have been labeled as experiencing high flu activity, while Idaho, Texas, Georgia, South Carolina, Michigan and Massachusetts were listed with moderate activity. The rest of the country has been characterized as low or minimal. influenza A accounts for about three of every four flu cases nationally, a pattern that has been increasingly associated with the spread of subclade K. [IMAGE1]

In the latest weekly update, the CDC also noted an eight percent increase in Americans testing positive for influenza compared with the prior week, underscoring the rapid pace of transmission in many communities. Officials cautioned that official figures could be higher once all data are compiled, and that the true scope of spread is likely undercounted by testing and reporting gaps.

The season has already claimed pediatric lives in at least one state. Colorado health officials said they had recorded the first flu-related pediatric death of the 2025-26 season, involving an elementary school-aged child who died last week. Officials stressed that it remains unclear whether this case is the same child referenced in other reports, but the outbreak underscores the seriousness of the current activity and the vulnerability of children.

Beyond the flu, other winter viruses are rising. Respiratory syncytial virus, or RSV, has shown substantial activity in several states and is being tracked in wastewater data as part of broader respiratory illness surveillance. In particular, wastewater signals indicate very high activity in Wyoming and Louisiana, high in Kentucky, and moderate levels in Alabama and Florida. RSV remains especially dangerous for infants and older adults; each year, tens of thousands of young children are hospitalized with RSV, and a smaller number die. Health experts emphasize that protection against RSV can be provided by a vaccine given to pregnant women, newborns and adults over 65, with estimates of about 80 percent effectiveness.

COVID-19 continues to circulate alongside flu and RSV. In some states, wastewater data show elevated levels of the virus, though the CDC has not tracked COVID test positivity, deaths or hospitalizations since late September. Connecticut, Nebraska and Vermont have reported high COVID activity in wastewater, with Indiana showing very high levels at 27 sites. Other states reported moderate activity, while several jurisdictions remained at very low levels. The dominant variant in wastewater data is identified as XFG, with patients reporting throat discomfort described as severe and sharp as part of the symptom set, though researchers caution that symptoms remain broadly similar to those seen with previous strains. Health officials say that, as with prior winters, COVID-19 tends to peak in late December and January.

Public health experts urge a layered approach to staying healthy this season, including vaccination against influenza and COVID-19, ongoing hand hygiene, staying home when sick and seeking care when symptoms worsen or fail to improve after a few days. Medical professionals such as Neal Shipley, medical director of Northwell Health-GoHealth Urgent Care, advise seeking urgent care promptly if there is trouble breathing, dehydration, severe weakness or rapidly worsening symptoms.

As the holidays approach, health officials caution that the convergence of multiple respiratory viruses can strain health care systems and complicate individual illness. People planning travel or large gatherings should be aware of local activity levels, monitor symptoms, and seek care if they belong to high-risk groups or develop warning signs. While vaccines remain a key defense, data suggest the current flu vaccine effectiveness ranges broadly, typically around 30 to 75 percent depending on strain match and other factors. Officials emphasize that vaccination remains the best available tool to reduce severe illness and death, even as new viral sublineages circulate.