A person is vaccinated against influenza, which is causing high numbers of illnesses and hospitalizations in the United States and elsewhere.Credit: H.Bilbao/Europa Press/Getty
As millions of bedridden people can attest, influenza is surging around the world. The virus has driven a wave of illness and hospitalizations in countries such as the United Kingdom, Italy and the United States, where “suddenly everybody is seeing not just cases, but high numbers of cases”, says Andrew Pekosz, a virologist at Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. In many nations, the flu season started earlier and accelerated faster than usual.
A universal flu vaccine has proved challenging — could it finally be possible?
So, why is this flu season so bad? Scientists suspect that it is in part due to a new strain of the influenza virus that has risen to dominance. The variant has a high number of key mutations, which means that it is much less similar to the strain used in the flu vaccine than previous seasons’ viruses have been. This might make it easier for the virus to shrug off the immune system and vaccines. Furthermore, the dominant strain belongs to a viral subtype that has been circulating for decades but was not dominant in the past few flu seasons, meaning that many people have relatively weak immunity against it.
Even so, there is evidence to suggest that currently available flu vaccines offer protection against severe illness1.
In the United States, it’s too early in the flu season “to say exactly how this one will stack up compared to [others] over the last few decades”, says Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle, Washington. But “this is a worse than average flu season, that’s for sure”.
A year for H3N2
The 2025–26 flu season started a month earlier than expected in the United Kingdom, much of Europe and Japan, which declared a flu epidemic because of the unexpectedly high number of infections. Australia’s flu season continued for at least a month longer than usual2. In Canada, “all provinces and territories saw a massive increase in the number of cases, all at the same time”, says Eleni Galanis, a director general at the Public Health Agency of Canada in Ottawa. “And that, of course, puts a lot of pressure on the health-care system.”
The virus causing many of this year’s cases is an example of the H3N2 subtype, which evolves faster than other strains3. A variant of the H3N2 virus called subclade K became globally dominant in September and now accounts for about 80% of influenza infections worldwide. “Everything can be attributed to this clade K variant,” Pekosz says.
Vaccine mismatch
Modelling suggests that subclade K emerged as early as February last year. It wasn’t sequenced until June — months after the World Health Organization selected the flu strains that were to be used as the basis of vaccines for the current northern hemisphere flu season. (Scientists adjust the vaccine’s composition every year to account for continual genetic changes in the virus.)
Immunological sin: how a person’s earliest flu infections dictate life-long immunity
Because of this timing, “there’s a mismatch between the vaccine strain and this circulating strain”, says Scott Hensley, a virologist at the University of Pennsylvania in Philadelphia.
Even so, in a preprint4 published on 6 January, Hensley and his colleagues found that in some people, the vaccine elicits enough antibodies against subclade K to protect against severe disease, Hensley says. The study has not yet been peer reviewed.
