On December 18, 2025, experts published an article in JAMA Network warning that a severe type of influenza A(H3N2) virus may become common this season. The H3N2 flu virus first emerged in 1968, and in that year's flu season, the H3N2 strain led to an estimated one million deaths worldwide. This year, there have been widespread reports of infections from an H3N2 flu virus that is known as subclade K. H3N2 flu viruses tend to evolve faster, cause more severe illness, and tend to have higher mortality rates in older adults than other types. This year’s vaccine, while still providing some protection against severe flu, is also not well-matched to the H3N2 strain. Vaccination is still important, however.
"Getting vaccinated as soon as possible remains the single most important action people can do to protect themselves and their families," said Frederick G. Hayden, MD, a professor emeritus at the University of Virginia School of Medicine. The flu vaccine is still recommended by experts for anyone over the age of six months old.
Now, multiple US states and the Centers for Disease Control and Prevention (CDC) have reported very high levels of flu activity in the United States. Two American children have also died from the flu so far this year, according to the CDC. The most recent data on wastewater surveillance, from December 18, 2025, shows that flu activity is considered moderate in seven states and high in Connecticut. Flu activity tends to peak between December and February.
Another study reported in BMJ has determined that this year’s flu season in the UK is a nasty one, which has also been fueled by the subclade K variant of H3N2. Japan and Taiwan also reported severe flu seasons. There has been a significant surge in flu cases in Australia as well; all of those surges are blmaed on subclade K, which is now being dubbed a 'superflu' by some media outlets.
"The United Kingdom got hit pretty hard by this," as did Europe and Australia, said Neil Maniar, director of the master of public health program at Northeastern University. "I think that's a pretty good harbinger of what we are likely to see here. This is going to be a very difficult flu season."
Sources: JAMA Network, BMJ, CDC, Northeastern University