Flu Season 2025-26

By: David Ostrowsky

It’s starting earlier this year.

And it may even be more complicated.

Generally speaking, flu season begins in earnest around Christmas and accelerates into the new year. But this year, the early days of the holiday season are coinciding with the pervasiveness of a new flu subvariant, H3N2 subclade K, that is flashing ominous warning signs for the looming winter. This particular strain, an offshoot of the influenza A (H3N2) virus, is particularly troublesome because it has seven new mutations that make it difficult for the immune system to recognize. In fact, it’s already descended on several countries—Japan reportedly issued a national alert with flu cases surging and hospitalizations increasing while England is experiencing its earlier start to the flu season since 2003-04—and may be spreading more easily than earlier versions of H3N2, according to the Centers for Disease Control and Prevention (CDC).

Though America has not yet borne the brunt of the new flu subvariant, the potential for a second consecutive severe flu season is real. (Last winter, the US had its highest rates of flu hospitalizations in nearly 15 years while at least 280 children died of influenza, the highest number since pediatric death numbers were required to be shared in 2004.) And this is largely due to the inauspicious timeline: because H3N2 subclade K emerged in late summer, health officials had already determined how to formulate this autumn’s flu vaccine. In other words, the available vaccine is not a perfect match for the latest mutated flu strain that has already wreaked havoc across continents. Even if subclade K doesn’t actually make a person sicker compared to other flu strains, it could still trigger a higher volume of cases, and thus hospitalizations. Physicians are naturally more concerned about how the new strain will impact the most vulnerable population, which includes babies, toddlers and young children up to age five, as well as those of an advanced age who have compromised immune systems.

All that being said, there is still reason for optimism that the new strain won’t take a devastating toll on America. Health officials have pointed out that just because H3N2 subclade K, whose core symptoms (fever, cough, sore throat, body aches, fatigue) remain typical of influenza, is spreading globally, it doesn’t mean that will be the case in the US. Ultimately, the severity of a given flu season depends on multiple factors including circulating strains, pre-existing immunity, vaccine uptake, and the overall health of the population. Though there’s inconclusive evidence as to whether subclade K will diminish the effectiveness of available flu shots, early scientific analysis does indicate that this year’s vaccines offer some protection against being hospitalized with the variant. According to the Centers for Disease Control and Prevention (CDC), even a partial vaccine match can greatly lower the risk of severe illness, hospitalization and complications.

As the CDC continues monitoring how widely and rapidly subclade K advances in the U.S.—in the first week of December Western Pennsylvania and Long Island have been hit very hard—and how well the flu vaccine performs against it, health officials are planning to release updated reports in the coming weeks.

Stay tuned.