Why This Year's Flu Season is So Bad: Understanding Subclade K

A lot of the talk about this year's flu season has centered on something called "subclade K." To understand what this is, it's important to know that flu viruses are constantly evolving, gaining small genetic mutations as they spread. Scientists keep track of how flu viruses change using a system similar to a family tree, Madad said. The big branches of this tree are called "types." This includes influenza A and B, which are broad groups of flu viruses. These big branches sprout off into smaller branches called subtypes, such as A(H3N2) or B/Victoria lineage. The letters H and N, which are used to differentiate influenza A subtypes, stand for two proteins on the virus's surface: hemagglutinin (H) and neuraminidase (N). There are many possible versions of each protein — 18 and 11 versions, respectively — which consequently gives rise to a large variety of flu viruses, of which only a few routinely infect humans. Going even further down a branch, scientists identify "clades," which are groups of viruses that came from a common ancestor and share similar genetic traits. A subclade is an even smaller group within a clade — a newer "twig" on the tree — that shares recent mutations. This flu season, the CDC reported that most flu cases are being caused by the influenza subtype A(H3N2). Of the more than 430 samples of influenza A(H3N2) virus collected since late September 2025, over 91% were identified as part of a closely related group called subclade K. The answer is no. Subclade K carries distinct mutations in parts of the virus that our immune systems usually recognize, Madad said. Because of this, our immunity from past flu infections or vaccinations is lower than it might be if the subclade looked more "familiar." This enables the virus to spread more easily, resulting in more cases and, as a result, more hospitalizations. However, that does not automatically mean subclade K is more deadly or causes more severe illness than other circulating flu viruses, Madad said. Current epidemiologic data show that subclade K does not lead to more severe disease than other seasonal flu strains than are spreading now, she noted. "Flu symptoms this year are similar to those seen in previous seasons," Abdelnour added. People with the flu commonly experience fever or chills, cough, nasal congestion, muscle and/or body aches, headaches and fatigue. In severe cases, flu infections can lead to complications such as sepsis and pneumonia. Adults 65 and up; young children, especially those under 2; pregnant people; and people with chronic conditions like asthma, diabetes or heart disease are at highest risk for severe flu complications. The flu vaccine "may be a less perfect match for subclade K" than it could be, Madad said, but she stressed that vaccination still plays a critical role in preventing serious illness. Abdelnour said that the annual flu vaccine reduces the risk of serious complications, such as sepsis, pneumonia and significant neurologic symptoms, such as seizures. Data from the Southern Hemisphere, which experiences flu season before the Northern Hemisphere, suggests that this year's flu vaccine works just as well as past vaccines at lowering the risk of being hospitalized with the flu, Abdelnour said. This past flu season, the flu vaccine reduced flu-related doctor visits and hospitalizations by 50% in the Southern Hemisphere, according to a CDC report published in September 2025. "This is within the typical range, and it's anticipated we should experience similar levels of protection in the United States," Abdelnour said. Unfortunately, fewer people are getting vaccinated against the flu this year, and that's especially true of children, Madad said. "Only about 42% of U.S. children have received the flu vaccine so far this season, down from roughly 53% at the same point in 2019–2020." As of Jan. 3, about 44.1% of adults reported having received a flu vaccination. Poor vaccination rates can have real consequences, Madad said. During the 2024-2025 flu season, 280 children in the U.S. died of flu-related illness, and of those, about 9 in 10 had not been vaccinated against the virus. Madad, who is also a mother of four, noted that all of her children receive the flu shot every year. Even though flu season started in the fall, "it is not too late to get the flu shot," Abdelnour emphasized. "Given the current surge in cases, getting vaccinated now is strongly recommended to protect yourself and those around you." Madad echoed that advice, urging anyone who hasn't yet received a flu shot this season to do so, especially adults ages 65 and older, pregnant people, those with weakened immune systems, and people living with chronic health conditions, who are at higher risk of severe cases. Vaccination is just one layer of protection. Madad emphasized wearing a well-fitting mask in crowded indoor spaces and improving ventilation for indoor gatherings. Additionally, people should stay home when sick to avoid the risk of spreading flu to others. These steps help limit the spread not only of flu, but of other respiratory viruses like COVID-19 and RSV (respiratory syncytial virus), as well. Abdelnour also noted the importance of practicing proper hand hygiene. "Special attention should be given to childcare environments such as schools and daycare centers," she said. "This includes recommending children get their flu shot, encouraging handwashing with soap and water for at least 20 seconds and keeping children home when they are ill to prevent the spread of infection to others."