This winter, the United States is recording a sharp rise in influenza cases. A significant share of infections is being linked to the so-called “super flu”—a term used in public debate to describe a particularly fast-spreading strain.
The flu surge is unfolding alongside a simultaneous increase in other seasonal illnesses—the “winter vomiting bug,” COVID-19, and whooping cough—and coincides with a period of institutional instability at the US Centers for Disease Control and Prevention, where disputes continue over childhood vaccination schedules. According to the CDC, influenza activity remains elevated nationwide.
Since the start of the current season, the United States has recorded at least 7.5 million flu cases, around 81,000 hospitalizations, and 3,100 deaths linked to influenza. A substantial proportion of these cases is attributed to the so-called subclade K—a variant of the H3N2 virus, which belongs to influenza A. Experts and patients alike use the term “super flu” to describe a strain that spreads especially rapidly and places greater strain on the healthcare system. The CDC, however, emphasizes that “severity indicators remain low at this time, although influenza activity is expected to continue for several more weeks.”
The rise in infections has spread across virtually the entire United States. In New York state, during the week ending December 20, a record number of positive flu tests was reported—the highest in the history of monitoring. In Connecticut, according to local media, infection levels have reached “the highest doctors have ever seen.” In California, case numbers are increasing statewide, and Dawn Terashita, a spokesperson for the Los Angeles County Department of Public Health, said it was “frightening to hear that this year could turn out worse than last.” At the same time, record figures are also being reported in Europe—particularly in England and Ireland—where masks and hygiene measures are once again being introduced in some regions to curb the spread of the virus. Some specialists are comparing the current flu wave in Europe to the COVID-19 pandemic, as hospitals, under pressure from an influx of patients, urge visitors to wear masks and observe precautionary measures.
The currently dominant H3N2 strain first emerged over the summer, and in the autumn health authorities in the United Kingdom and Canada warned that it was driving an increase in hospitalizations. Experts’ concerns stem from the fact that the current flu vaccine targets the “2024–25 season subclade J” and the “updated 2025–26 season subclade J.2,” while it is subclade K that is now fueling the spread of H3N2. According to specialists, this variant has seven mutations compared with others. At the same time, a mismatch between the vaccine and the dominant strain is considered relatively common, and data from UK authorities indicate that the vaccine nevertheless provides some degree of effectiveness against H3N2.
There is no official medical term known as “super flu.” According to specialists, the label tends to emerge whenever a strain more aggressive than usual is in circulation. This season, it is most often associated with subclade K. According to the latest CDC data, nearly 90% of new flu cases in the United States are attributed to this variant. Amesh Adalja, an infectious-disease specialist at Johns Hopkins University, has described “super flu” as “a sensationalist term with no real medical meaning.” He explained that weak population-level immunity and incomplete coverage by the current vaccine leave many people vulnerable, but that “there is no indication this variant is more severe than other strains,” and that the vaccine—as well as antiviral drugs—continues to protect against severe illness. Andrew Pekosz, a virologist at the same university, noted that similar viruses have circulated before and said he would not label them a super flu either. At the same time, he cautioned that “rapid spread can result in a large number of flu cases over a short period of time, overwhelming medical centers.”
The symptoms associated with H3N2 do not differ from those of typical influenza—sore throat, runny nose, fever, cough, headache, muscle aches, fatigue, sweating, and chills. Unlike a common cold, which develops gradually, influenza, as the Mayo Clinic notes, usually begins abruptly—two to three days after exposure to the virus.
Specialists recommend that those who have not yet been vaccinated do so. Developing an immune response after vaccination takes about 10–14 days and can either prevent infection or reduce the severity of illness. According to Pekosz, the vaccine provides strong protection against H1N1 and H3N2 and offers “a certain degree of protection against severe disease caused by subclade K.”