What we know about flu variant A/H3N2 K: The strain behind 90 percent of cases

Every winter, respiratory viruses, especially influenzaSARS-CoV-2 and RSV, put enormous pressure on health care systems Continuous EuropeIn a normal season, Fever There are 50 million symptomatic cases and 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 contract the disease and spread it within their homes. It is estimated that up to 20% of the population is infected Fever Every year.

There are two main types of flu viruses that commonly infect humans: influenza A (the most variable, responsible for most seasonal epidemics and all known flu epidemics) and influenza B (which spreads almost exclusively among humans).

there are many different subtypes Of influenza A, it depends on how their hemagglutinin (H) and neuraminidase (N) proteins combine. The subtypes currently circulating in humans are H1N1 (including the 2009 pandemic variant, which is now seasonal) and H3N2These subtypes may vary from season to 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.

Why is there variability in flu viruses? Vaccines are produced every year With a mixture of three (or four) viruses that circulated last year.

What’s happening this autumn?

Flu season started a few weeks ago Compared to previous years. Japan, for example, has been reporting an unusually high number of cases since October.

Flu is emerging unusually quickly in the EU. In Spain, for example, the limits of the epidemic have been crossed a few weeks earlier than expected.

Another new development is the apparent predominance of A/H3N2 viruses, especially subtype K. Although A/H1N1 viruses predominated in the Southern Hemisphere this year and subtype K of A/H3N2 only finally spread, it became dominant in the United Kingdom and Japan: about 90% of flu samples in these countries are now A/H3N2 K. The United States and Canada are following a similar trend.

Subclade K of A/H3N2 (formerly known as J.2.4.1) has now been found on all continents. The virus accounted for one-third of all A/H3N2 viruses analyzed between May and November 2025 worldwide and nearly half in the European Union.

Subclass K: Different, but not necessarily more harmful

Genetic changes in subclade K of A/H3N2 viruses do not result from recombination or mixing of viruses, and do not represent any major changes. It is actually the same H3N2 virus that has been spreading among humans since 1968, but with some mutations in its genes.

This is to be expected, because each season the flu virus evolves (through mutation) to evade our immune systems. In some years it is more successful than others.

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Mutations in this new subclass do not affect the efficacy of antiviral drugs, nor are they associated with increased toxicity or disease severity.

In fact, while East Asian countries are now reporting a decline in subtype K cases, they have not experienced unusually high severity of the disease. Furthermore, the analysis shows that the subtype K strains circulating in these countries do not differ from the strains currently present in the EU. So, the good news is that subtype K is not more virulent or causes more severe disease.

Does this mean more cases?

A/H3N2 has not been the dominant influenza virus in recent seasons – this has usually been A/H1N1. This may lead to reduced immunity in the population due to lack of recent exposure to A/H3N2.

flu symptoms

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Additionally, seasons dominated by this pathogen are more severe, with lower vaccine efficacy and more severe cases among older adults than seasons dominated by A/H1N1.

vaccine safety

To make flu vaccines, scientists use data from the previous year to predict which strains of the virus are likely to be most prevalent in the coming year. This season’s vaccine was designed to protect against two subtypes of influenza B, A/H1N1, and a type A/H3N2 that predates the new subtype.

Some analyzes show 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 wanes, we can expect more cases of flu. Monitoring this season is important to determine how much protection the vaccines are providing.

However, even if a less vaccine-matched A/H3N2 virus becomes dominant this winter, vaccination is still expected to provide protection against severe disease and so remains an essential public health tool.

Currently, the risk appears to be moderate for the general population, but the risk is higher for people who are more likely to develop severe disease (primarily people over 65, those with other diseases, pregnant women, or those who are immunocompromised).

So, although subclade K of the A/H3N2 virus does not appear to be more virulent, the fact that flu season arrived a few weeks ago and the most prevalent strain is A/H3N2 suggests that this will be a more difficult season, with higher numbers of hospitalizations and greater pressure on health services.

My recommendation: Get vaccinated as soon as possible

Even in seasons when the vaccine is less effective, it still provides some protection, and it is the most effective weapon we have to reduce the risk of severe disease.

For all these reasons, the vaccine is recommended for all people who are eligible. Additionally, this year’s flu pandemic arrived early, and protection against the virus takes a few weeks to become fully active after receiving vaccination. This means that you should get the vaccine without delay.

Finally, it is important to stress the importance of washing hands and wearing a mask if you suspect you are infected. This will prevent the spread of infection to those who are most vulnerable.