Flu cases in the U.S. took a sharp rise over the holidays. By the end of December 2025, weekly hospitalizations jumped from under 7,000 to over 33,000. The Centers for Disease Control and Prevention (CDC) estimates that more than 11 million people have been hit by the flu this season, with over 5,000 deaths reported.
So, what’s driving this surge? Experts point to a new influenza A mutation called “subclade K.” This variant isn’t well covered by the current flu vaccine. Each year, scientists try to predict which flu strains will circulate, but sometimes the viruses mutate unexpectedly, as they have this year.
Even with the vaccine’s limitations against subclade K, getting vaccinated is still worth it. Experts agree that the vaccine can reduce the severity of illness and protect against other circulating flu strains. Dr. Caitlin Li from Lurie Children’s Hospital says, “Vaccinated individuals experience lower rates of hospitalization and death, even if the vaccine isn’t a perfect match.”
Each year, there are two main types of influenza – A and B. Both can mutate into different subtypes. The vaccine’s formulation needs to be decided months in advance, which leaves room for new strains to develop. Dr. Buddy Creech from Vanderbilt notes, “We do all we can to predict which strains will dominate, but sometimes we are caught off guard.”
This year’s vaccine was designed to protect against three specific strains: two types of influenza A (H1N1 and H3N2) and one type of influenza B (Victoria lineage). However, after the formulation was set, the H3N2 strain mutated, causing a mismatch. Andrew Pekosz, a professor at Johns Hopkins, explains that this type of mismatch happens every couple of years as the flu virus continues to evolve.
Despite the mismatch with the H3N2 strain, the vaccine still protects against H1N1 and influenza B. “We often see different strains causing illness later in the season,” Pekosz adds. So, if you haven’t been vaccinated yet, it might still be beneficial, especially if you’re at high risk for severe complications.
The term “super flu” has emerged, but it might be misleading. While the subclade K variant is better at evading the vaccine’s protection, there’s no evidence suggesting it is more severe or contagious. The CDC reports that flu vaccination rates are lower than ideal, at around 40% this season. This contributes to the unusual intensity of the current flu outbreak.
Getting vaccinated remains a key defense against not just the prevalent strains but also future variants. Experts agree on its importance, highlighting that even with challenges, vaccination plays a crucial role in public health during flu season.
