This article is republished from The Conversation under a Creative Commons license.
Every winter, respiratory viruses, especially influenza, SARS-CoV-2 and RSV, put huge pressure on healthcare systems throughout Europe. In a typical season, flu causes up to 50 million symptomatic cases, and anywhere from 15,000 to 70,000 deaths per year on the continent.
All age groups are affected, although children have the highest infection rates and are often the first to catch the disease and spread it in their homes. It is estimated that up to 20% of the population contracts flu each year.
The flu viruses that normally infect humans belong mainly to two types: influenza A (the most variable, responsible for most seasonal epidemics and all known flu pandemics) and influenza B (which circulates almost exclusively in humans).
There are many different subtypes of influenza A, depending on how their haemagglutinin (H) and neuraminidase (N) proteins combine. The subtypes currently circulating in humans are H1N1 (including the 2009 pandemic variant, which is now seasonal) and H3N2. These subtypes can vary each season due to small mutations or antigenic drift.
Influenza B has two lineages: B/Victoria and B/Yamagata. B/Yamagata has hardly been detected since 2020.
The flu virus’ variability is why vaccines are prepared each year with a mix of three (or four) of the viruses that circulated the previous year.
What’s Happening This Autumn?
The flu season has started a few weeks earlier than in previous years. Japan, for instance has been recording an unusually high number of cases since October.
In the European Union, flu is appearing unusually early. In Spain, for instance, the epidemic threshold has already been exceeded, a few weeks earlier than expected.
Another new development is the clear predominance of the A/H3N2 virus, specifically the subclade K. Although the A/H1N1 virus predominated this year in the southern hemisphere and the subclade K of A/H3N2 only took off at the end, it ended up being predominant in the United Kingdom and Japan: nearly 90% of flu samples in these countries are now A/H3N2 K. The United States and Canada seem to be following a similar trend.
Subclade K of A/H3N2 (formerly known as J.2.4.1) has now been detected on all continents. It accounted for one-third of all A/H3N2 viruses analyzed between May and November 2025 worldwide, and almost half in the EU.
Subclade K: Different, but Not Necessarily More Harmful
The genetic changes in subclade K of the A/H3N2 virus do not come from recombination or mixing of viruses, and do not represent a major change. It is actually the same H3N2 virus that has been circulating among humans since 1968, but with some mutations in its genes.
This is to be expected, as each season the flu virus evolves (through mutation) to escape our immune systems. Some years it is more successful than others.
Mutations in this new subclade do not appear to affect the efficacy of antiviral drugs, nor are they associated with increased virulence or disease severity.
In fact, the East Asian countries now reporting a decline in subclade K cases have not experienced unusually high severity of illness. Furthermore, analyses suggest that the subclade K strains circulating in these countries do not differ from those currently present in the EU. The good news, therefore, is that subclade K does not seem to be more virulent or cause more severe disease.
Does This Mean More Cases?
A/H3N2 has not been the dominant influenza virus in recent seasons—it has usually been A/H1N1. This could lead to lower immunity in the population due to a lack of recent exposure to A/H3N2.
In addition, seasons dominated by this pathogen tend to be more severe, with lower vaccine efficacy and more serious cases in older adults than in seasons dominated by A/H1N1.
Vaccine Protection
To create flu vaccines, scientists use data from the previous year to predict which strains of the virus are likely to be most prevalent over the coming year. This season’s vaccine was designed to protect against two subtypes of A/H1N1, influenza B, and a type of A/H3N2 that predates the new subclade K.
Some analyses show a divergence of subclade K from the vaccine strain of H3N2, although data on vaccine effectiveness in the real world is currently limited. If vaccine efficacy is reduced, we can expect more cases of flu. Monitoring this season is crucial to determine how much protection vaccines are providing.
However, even if a less vaccine-matched A/H3N2 virus becomes predominant this winter, immunization is still expected to provide protection against severe illness and therefore remains an essential public health tool.
At present, the risk to the general population appears to be moderate, but higher for people who are more likely to develop severe disease (mainly those over 65, with other illnesses, pregnant women, or those who are immunocompromised).
Therefore, although subclade K of the A/H3N2 virus does not appear to be more virulent, the fact that the flu season has arrived a few weeks early and that the most prevalent strain is A/H3N2 suggests that this will be a more difficult season, with a higher number of hospitalizations and greater pressure on health services.
Recommendation: Get Vaccinated ASAP
Even in seasons when the vaccine is less effective, it still offers some protection, and it is the most effective weapon we have for reducing the risk of serious illness.
For all these reasons, the vaccine is recommended for everyone who is eligible. Furthermore, this year’s flu epidemic has arrived early, and it takes a few weeks after receiving the immunization for defenses against the virus to become fully active. This means you should get the vaccine without delay.
Lastly, it is important to emphasize the importance of hand washing and wearing a mask if you suspect you are infected. This will prevent the spread of infection to those who are most vulnerable.
Ignacio López-Goñi, Catedrático de Microbiología. Miembro de la Sociedad Española de Microbiología (SEM), Universidad de Navarra
This article is republished from The Conversation under a Creative Commons license. Read the original article.
