A powerful new strain of influenza, dubbed a 'super flu', is wreaking havoc across the United States, driving infection rates to unprecedented levels in major cities and leaving some children so severely ill they require mechanical ventilation to breathe.
Unprecedented Spread of a Novel Virus
The culprit is a previously unseen variant known as subclade K, a mutation of the influenza A H3N2 subtype. Data from the US Centers for Disease Control and Prevention (CDC) for the week ending December 27 reveals a startling escalation. The number of states reporting 'very high' influenza activity has jumped from 22 to 32 in just one week.
Laboratory findings are equally alarming, with one in three flu tests now returning positive, a significant increase from one in four the previous week. This represents a 76 percent surge compared to the same period last year. New York City reported over 72,000 cases in the week of December 20 alone, a record high for the region.
Global Journey of the K Strain
Health authorities often look to the Southern Hemisphere's winter as a predictor for Northern flu seasons. This year, Australia's season was unusually prolonged and severe, with the Doherty Institute for Infection and Immunity noting it 'differed from what is typically observed.' A study in Eurosurveillance detailed how a fast-spreading H3N2 variant emerged late in their season, becoming a leading cause of respiratory deaths by August.
Intriguingly, genomic detective work suggests this dominant strain may have originated in the United States before circling the globe. The first sequenced K virus was detected in New York in June 2025, followed by Wisconsin and Michigan in July. This timeline coincided with the first detections in Australia, leading researchers to hypothesise the strain was exported from the US, fuelled a severe Southern Hemisphere season, and has now returned.
Subclade K viruses now account for roughly half of Australian flu samples and over two-thirds in New Zealand. In the US, an overwhelming 91 percent of H3N2 viruses analysed belong to this new subclade. It has been detected in at least 34 countries worldwide.
Severe Symptoms and Strain on Healthcare
Doctors report this new strain presents with distinct and often severe symptoms. Infections frequently begin with a stubbornly high fever that can last five to seven days and may not respond well to common remedies like paracetamol or ibuprofen. This progresses to a phlegmy, persistent cough, extreme fatigue, vomiting, diarrhoea, and significant muscle and joint aches.
In serious cases, patients struggle to breathe, requiring ventilator support. The case of two-year-old Sarah Lopez from Georgia illustrates the danger. Hospitalised with influenza A, she developed a rare spinal cord inflammation called transverse myelitis. She was placed on a ventilator and feeding tube, and while now improving, faces a long rehabilitation.
The human cost is mounting. This season, the CDC estimates at least 11 million illnesses, 120,000 hospitalisations, and 5,000 deaths from flu, figures that have roughly doubled from last year. Nine paediatric deaths have been confirmed, including four in Massachusetts and one in Illinois.
Experts warn that the novelty of subclade K likely reduces the effectiveness of the current seasonal flu vaccine. A recent study cautioned that countries must prepare for increased pressure on healthcare systems, as this variant is expected to predominate throughout the 2025-2026 Northern Hemisphere winter.
