UNIPA News,02 June-A newly emerged influenza A (H3N2) variant known as subclade K has become the dominant flu strain in the United States, contributing to a sharp rise in infections, hospitalizations, and severe respiratory illnesses during the current flu season.
According to the latest data from the U.S. Centers for Disease Control and Prevention (CDC), 32 states reported “very high” influenza activity for the week ending December 27, up from 22 states the previous week. The agency reported that approximately one in three flu tests returned positive results, representing a significant increase from the previous week and a substantial rise compared with the same period last year.
Health officials estimate that the United States has recorded at least 11 million influenza illnesses, 120,000 hospitalizations, and 5,000 deaths so far this season. During the same period last year, the CDC estimated 5.3 million illnesses, 63,000 hospitalizations, and 2,700 deaths.
Virological surveillance indicates that influenza A accounts for 97 percent of flu samples collected since September. Of those samples, 86 percent were identified as H3N2 viruses, while 91 percent of H3N2 cases belonged to the newly dominant subclade K lineage.
Researchers monitoring influenza activity in Australia and New Zealand reported that subclade K spread rapidly across both countries during their most recent flu season. A study published in the journal Eurosurveillance found that H3N2 viruses overtook H1N1 strains during the latter part of the southern hemisphere season and became the dominant influenza subtype.
The study reported that subclade K viruses accounted for roughly half of influenza cases analyzed in Australia and more than two-thirds of those detected in New Zealand. Researchers warned that the speed and scale of the outbreaks suggested the variant could continue expanding throughout the northern hemisphere winter season.
Genomic analysis cited in the study indicated that the earliest sequenced subclade K virus was detected in New York in June 2025, followed by additional detections in Wisconsin and Michigan. Researchers suggested the strain may have spread internationally before contributing to major outbreaks in Australia and New Zealand.
Scientists have now identified subclade K viruses in at least 34 countries. Researchers said the variant’s rapid global spread could place additional pressure on healthcare systems if it remains the dominant circulating strain.
The study also noted that the novelty of subclade K may reduce the effectiveness of existing seasonal influenza vaccines compared with previously circulating strains, potentially increasing the risk of infections and hospitalizations.
Doctors across the United States have reported that many patients infected with the strain are experiencing unusually high fevers, persistent coughs, severe fatigue, muscle aches, joint pain, and, in some cases, gastrointestinal symptoms such as vomiting and diarrhea. Medical experts have also reported severe respiratory complications requiring hospitalization and mechanical ventilation in some patients.
Public health authorities continue to monitor the spread of the strain as the winter flu season progresses.