Meet Subclade K: What This Year's Flu Means for the ED

The 2024-25 U.S. flu season was severe, with a hospitalization rate of 128.3 per 100,000, the highest since 2010-11.1 And as the 2025-26 flu season roiled on, the headlines were once again dire. As of the week ending January 24, the Centers for Disease Control and Prevention (CDC) recorded at least 20 million illnesses, 270,000 hospitalizations, and 11,000 influenza-related deaths—52 of them pediatric—this season.2 One reason for the unusual pattern of back-to-back severe seasons is the predominance of influenza A (H3N2), which tends to cause more severe disease, said Robert H. Hopkins, Jr, MD, medical director of the National Foundation for Infectious Diseases (NFID). In addition, a new variant of H3N2—subclade K—emerged in June, after the World Health Organization (WHO) had chosen the recommended strains for this year's influenza virus.3 Consumer publications branded subclade K a “superflu,” but infectious disease experts were more restrained in their descriptions.4 The WHO reported that while K marks a notable evolution in influenza A(H3N2) viruses, current epidemiological data do not indicate an increase in disease severity.5 Internal genetic changes that define an influenza subtype may be but aren't necessarily linked to differences in how the illness looks to physicians or feels to patients [the externally-apparent phenotypic profile] such as infectiousness, ease of transmission, or any other change in how the virus behaves, said Charles Bailey, MD, medical director for infection prevention at Providence St. Joseph Hospital and Providence Mission Hospital. This differs from diseases like Mpox and Ebola, where new clades had distinct clinical presentations or treatment responses. “We don't really have enough evidence to say yet whether this is going to be more severe than any other H3N2, but it is different,” Dr Hopkins said. “That means that people are less likely to have any existing immunity that's going to protect them.” He notes that while this year's vaccine isn't a perfect match, it still provides significant protection, particularly against severe outcomes. Early analyses from England published in the journal Eurosurveillance suggest this year's vaccine was 72%-75% effective against visits to the emergency department (ED) and hospitalizations in children and adolescents, and 32%-39% in adults, which are typical ranges.6 But vaccination is only effective if it's utilized, and low rates may also play a role in influenza prevalence and severity. As of December 27, 2025, 43.5% of adults and 42.5% of children had received the 2025-26 vaccine.7 Emergency physicians should continue to emphasize the importance of vaccination, even late in the season or after recovery from influenza illness, Dr Hopkins said. While further investigations will shed more light on subclade K's characteristics, clinicians haven't seen differences in the presentation or progression of individual cases of influenza. “There's no specific identifying symptom complex unique to this—it's the fevers, the body aches, the myalgias and fatigue, along with nasal symptoms, sore throat, and cough,” said Jerry Chiricolo, MD, chair of the emergency department at Community Medical Center in Toms River, New Jersey. “It's the typical flu presentation, we're just seeing a lot more of it.” As in previous seasons, emergency physicians (EPs) should also stay alert for atypical influenza symptoms in certain populations, including those who are immunocompromised or at the extremes of age. “Often, very young children will present with just irritability,” Dr Hopkins said. “Older adults often present with lassitude, malaise, or confusion.” Treatment recommendations for subclade K are the same as for any other case of influenza. The same antiviral medications—including oseltamivir, zanamivir, and baloxavir—remain effective for treating and preventing influenza in high-risk patients and should be started as soon as possible.8 Fortunately, there's no evidence of antiviral resistance associated with subclade K, Dr Hopkins said. While some testing is important for surveillance, high-risk patients with influenza-like illness can be treated empirically to speed treatment and preserve resources in a busy ED. “When you're dealing with the sheer volume that we are right now, you have to be a little judicious about when and who you test,” Dr Chiricolo said. At his facility, EPs are using a 4-in-1 point-of-care test that detects SARS-CoV-2, influenza A/B, and RSV for many patients, reserving laboratory testing mostly for those at very high risk or who are being admitted. As of mid-January, it wasn't yet clear how long the U.S. flu season would last. The season was prolonged in some places in the southern hemisphere, including Australia, but had begun to show mixed trends in the United Kingdom after a severe and early start.9 However long the season endures, EPs and other clinicians can continue to encourage vaccination and reassure patients alarmed by news reports that little has changed for most people. “Influenza has always been a complex, continually changing virus, which we have many decades of experience in preventing, diagnosing, and treating,” Dr Bailey said. “People at risk for severe flu will do better with vaccinations, and if symptomatic flu does arise, their health care providers will be able to employ the same tried and true methods of diagnosis and treatment.”