Flu season is beginning to intensify across the United States as colder weather and more time spent indoors create conditions that can accelerate the spread of respiratory viruses. Data from the Centers for Disease Control and Prevention shows that at least four states — Colorado, Louisiana, New Jersey, and New York — reported high levels of influenza-like illness for the week ending Dec. 6. During that same period, 8.1% of flu tests returned positive results, a sharp increase from the 1.6% positivity rate recorded just one month earlier. Hospitalizations are also rising, with more than 6,800 flu-related admissions reported nationwide in that week alone, alongside the first pediatric flu-related death of the season.
Health officials note that flu season has begun earlier in some areas, particularly in New York, where state health data shows more than 24,000 cases and nearly 1,400 hospitalizations for the week ending Dec. 6. By comparison, the same week last year saw roughly half that number of cases and fewer hospital admissions. While flu activity typically peaks later in winter, usually January or February, early indicators suggest a more accelerated start this year. “If we look at what’s happening around the world, for example, the U.K., and what has happened in Japan and other parts of Asia, we’re worried that this may be yet another bad flu season,” Dr. Peter Chin-Hong said.
One factor drawing attention is the emergence of a newer influenza strain known as subclade K, a variant of the H3N2 subtype of influenza A. Public health experts report that this strain has been a primary driver of increased flu activity in countries including Canada, Japan, and the United Kingdom. According to the CDC, among 163 genetically analyzed H3N2 samples collected since late September, 89% were identified as subclade K. Researchers are still gathering data on whether the strain leads to more severe illness, but early indications suggest it may evade existing immunity more effectively than prior strains.
Dr. Tony Moody explained that while firm conclusions are still developing, subclade K appears to have found a pathway to spread more efficiently. “We think that this new circulating dominant strain probably is one that has just found a little bit of a hole,” he said. “In some ways. It’s almost like a NASCAR race, right? It found a slot that it could zoom through. But it’s going to take time to get firm data on that and really be able to say that for absolute sure.” Despite these uncertainties, data from the United Kingdom indicates that current flu vaccines remain effective at reducing severe outcomes, particularly hospitalizations.
A pre-print analysis from the U.K. Health Security Agency suggests the 2025–26 flu vaccine is between 70% and 75% effective at preventing hospitalization in children ages 2 to 17, and between 30% and 40% effective in adults. Chin-Hong emphasized that even with genetic changes in the virus, vaccination continues to reduce the risk of serious illness. “To me, even though they are about seven new mutations in subclade K, it means that if you get the vaccine, you’re going to diminish your chances of getting seriously ill,” he said. “But if you don’t get the vaccine, because there’s so many [new mutations], you may get sick more easily compared to, say, even last year.”
Federal health guidance continues to recommend flu vaccination for most individuals six months and older, alongside common-sense precautions such as frequent handwashing, covering coughs and sneezes, and limiting contact with others when ill. Health experts also note that early diagnosis can be important, as antiviral treatments may help reduce the severity and duration of illness. As flu activity increases, officials stress the importance of monitoring symptoms, particularly among children, older adults, and those with underlying health conditions, as healthcare systems prepare for a potentially demanding winter season.
