Compared to Previous Years and Important Prevention Tips
Doctors and health experts explain that with the arrival of each winter, respiratory viruses, especially influenza, SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus), and RSV (Respiratory Syncytial Virus), place immense pressure on healthcare systems in many countries worldwide.
Influenza causes up to 50 million symptomatic cases and between 15,000 and 70,000 deaths annually in Europe alone, according to the British newspaper The Independent.
Highest Infection Rates
All age groups contract influenza, although children have the highest rates and are often the first to be infected, spreading the disease within their homes. It is estimated that up to 20% of the world's population contracts influenza annually.
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Influenza A and B
The influenza viruses that typically infect humans belong to two main types: influenza A (the most diverse, responsible for most seasonal outbreaks and all known influenza pandemics) and influenza B (which circulates almost exclusively among humans). There are many different subtypes of influenza A, based on how the hemagglutinin (H) and neuraminidase (N) proteins combine.
Although the subtypes currently circulating among humans are H1N1 (including the 2009 pandemic variant, which is now seasonal) and H3N2, these subtypes can vary each season due to minor mutations or antigenic drift. Influenza B has two main strains: B/Victoria and B/Yamagata, but the B/Yamagata strain has been rarely detected since 2020. The diversity of the influenza virus is why vaccines are prepared each year using a mixture of three (or four) viruses that circulated in the previous year. An Early Flu Season
It has become noticeable that the flu season started a few weeks earlier than in previous years. For example, Japan has been recording an unusually high number of cases since October.
In the European Union, the flu is appearing unusually early. In Spain, for example, the epidemic threshold has already been crossed, a few weeks earlier than expected.
A Globally Dominant Substrain
Another new development is the clear dominance of the A/H3N2 virus, specifically the K substrain. Although the A/H1N1 virus was dominant this year in the Southern Hemisphere, and the K substrain of A/H3N2 only emerged towards the end of the season, it has become dominant in the UK and Japan, where nearly 90% of flu samples in these countries are now of the A/H3N2 K substrain. The US and Canada appear to be following a similar trend.
Genetic Mutations in the K Strain
The K sub-strain of A/H3N2, formerly known as J.2.4.1, has now been detected on every continent. It accounted for one-third of all A/H3N2 viruses analyzed globally between May and November 2025, and nearly half of those in the European Union. The K sub-strain differs from other strains, but it is not necessarily more dangerous.
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The genetic changes in the K sub-strain of the A/H3N2 virus are not due to recombination or viral mixing, and they do not represent a significant change. It is, in fact, the same H3N2 virus that has been circulating among humans since 1968, but with some mutations in its genes. It is normal for the influenza virus to evolve each season (through mutations) to overcome human immune systems, but it is more successful in some years than others.
In fact, East Asian countries, which are currently reporting a decline in cases of the K sub-strain, have not experienced unusual disease severity. Furthermore, analyses indicate that the K subtype strains circulating in these countries are no different from those currently present in the European Union. Therefore, the good news is that the K subtype does not appear to be more virulent or cause more severe illness.
Influenza A/H3N2 has not been the dominant influenza virus in recent seasons—it has usually been A/H1N1. The recent surge in cases is attributed to a decrease in immunity in the population due to less recent exposure to A/H3N2.
Symptoms of Influenza
Symptoms of influenza appear very quickly and can include:
- Sudden high fever
- Body aches
- Fatigue or exhaustion
- Dry cough
- Sore throat
- Headache
- Difficulty sleeping
- Loss of appetite
- Diarrhea or abdominal pain
- Nausea and vomiting
In addition, seasons when this virus circulates tend to be more severe, with less vaccine effectiveness and higher risk of severe illness among the elderly compared to seasons when A/H1N1 circulates.
Decreased Vaccine Effectiveness
To develop flu vaccines, scientists use data from the previous year to predict which virus strains are likely to be most prevalent in the coming year. This season's vaccine is designed to protect against two subtypes of the A/H1N1 virus: influenza B and an A/H3N2 variant that precedes the new K subtype.
Some analyses show a difference between the K subtype and the H3N2 vaccine strain, although data on the vaccine's actual effectiveness is not currently available. If the vaccine's effectiveness decreases, more flu cases can be expected.
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Risk Levels
Currently, the risk level for the general population appears to be moderate, but it is higher among those most vulnerable to severe illness (especially those over 65, those with underlying health conditions, pregnant women, or those with weakened immune systems).
Therefore, although the K subtype of the A/H3N2 virus does not appear to be more virulent, the fact that the flu season started a few weeks ago and that the most prevalent strain is A/H3N2 suggests that this season will be more challenging, with a rise in hospitalizations and increased pressure on healthcare services.
It is therefore advisable to get vaccinated as soon as possible, because even in seasons when the vaccine is less effective, it still offers some protection and remains the most effective weapon available to reduce the risk of severe illness.
Doctors and health experts also recommend consuming foods rich in fiber and vitamin C, such as kiwi, oranges, tangerines, lemons, warm beverages, and chicken soup.
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