What is the 'super flu' variant?

Last summer, a new mutated strain of influenza A (H3N2) called subclade K emerged in Australia. Later in the fall, health officials in the United Kingdom and Canada reported that subclade K was driving an increase in flu hospitalizations in their countries.

According to an early analysis from the U.K. Health Security Agency, subclade K has seven gene changes on an important segment of the flu virus. The mutations change the shape of the region, which makes it harder for the body's immune system to recognize.

Since early December, H3N2 subclade K has been the dominant flu strain in the United States. It is the first time since the 2016-2017 flu season that an H3N2 virus has been dominant.

Because of how rapidly subclade K has spread in the United States and other countries, some people have dubbed subclade K a "super flu." However, there is currently no evidence to suggest that it causes more severe illness or is more contagious than previous flu variants.

"While we expect more flu cases this year, we aren't sure if this virus causes more severe disease on a per case basis yet," said Andrew Pekosz, a virologist at Johns Hopkins University. "However, it is spreading so quickly that the increased numbers of influenza cases it causes can swamp a medical center with cases quickly."

Flu hospitalizations have also increased in recent weeks. For the week ending Dec. 27, the weekly hospitalization rate was 8.4 per 100,000 people, up from 7.6 per 100,000 the week before. In total, over 33,000 patients were hospitalized with the flu during the week ending Dec. 27.

So far this season, CDC estimates that there have been at least 11 million flu cases, 120,000 hospitalizations, and 5,000 deaths. For the week ending Dec. 27, two pediatric deaths from the flu were reported, bringing the current total for the 2025-2026 season to nine.

Although this year's flu shot does not specifically target subclade K, health experts are urging people to get vaccinated if they haven't already, saying that the shot will still help protect them against severe illness.

"It may mean that more people will be susceptible to getting the flu … but we know from historical data the vaccine is still very effective at preventing severe illness, hospitalization and death," said Molly Fleece, an associate professor in the division of infectious diseases at the University of Alabama at Birmingham.

"It's definitely not too late to get a flu vaccine if you haven't done it already," said Lisa Grohskopf, a medical officer in CDC's influenza division.