A person is vaccinated against influenza, which causes large numbers of illnesses and hospitalizations in the United States and elsewhere.Credit: H Bilbao/Europe Press/Getty
As millions of bedridden people can attest, influenza is spreading around the world. The virus has led to a wave of illness and hospitalizations in countries like the United Kingdom, Italy, and the United States, where “suddenly everyone was seeing not just cases, but large numbers of cases,” says Andrew Piekosh, a virologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. In many states, the flu season started earlier and accelerated faster than usual.
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So, why is flu season so bad? Scientists suspect that this is partly due to A A new strain of influenza virus has become dominant. The variant has a large number of key mutations, meaning it is much less similar to the strain used in the flu vaccine than to viruses from previous seasons. This may make it easier for the virus to get rid of the immune system and vaccines. Furthermore, the dominant strain belongs to a viral subtype that has been circulating for decades but has not been prevalent in the past few flu seasons, meaning many people have relatively weak immunity to it.
However, there is certificate To point out that currently available influenza vaccines provide protection against severe disease1.
In the United States, it’s too early to see a flu season “to say exactly how this season will stack up compared to the flu season.” [others] “Over the past few decades,” says Jesse Bloom, a virologist at Fred Hutchinson Cancer Center in Seattle, Washington. But “this is a worse-than-average flu season, that’s for sure.”
General for H3N2 virus
The 2025-2026 influenza season started a month earlier than expected in the United Kingdom, most of Europe and Japan, which declared an influenza pandemic due to an unexpectedly large number of infections. Australia’s flu season has lasted at least a month longer than usual2. In Canada, “all the provinces and territories saw a huge increase in the number of cases, all at the same time,” says Eleni Galanis, director-general of the Public Health Agency of Canada in Ottawa. “This 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 subtype K became dominant globally in September and now accounts for about 80% of influenza infections worldwide. “Everything can be attributed to this type of K,” says Pekosh.
Vaccine mismatch
Modeling suggests the emergence of subclass K As early as February last year. It was not sequential Until June – After months The World Health Organization has selected influenza strains Which was to be used as the basis for vaccines for the current flu season in the Northern Hemisphere. (Scientists adjust the vaccine composition every year to take into account ongoing genetic changes in the virus.)
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Because of this timing, “there is 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 the pre-print version4 Hensley says the vaccine, which was published on January 6, found that in some people, the vaccine produces enough antibodies against subclass K to protect against severe disease. The study has not yet been peer-reviewed.