A new subtype of the flu virus, known as influenza K, has attracted the attention of health authorities. Last week, the World Health Organization (WHO) warned of a “rapid growth” in cases of this subtype since August in different countries.
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Although, in general, infections are proceeding as expected for the season, with the arrival of winter next week in the Northern Hemisphere, the WHO says that in some regions of the planet there are increases above average and which started earlier.
“Some countries have reported an early start to the influenza season. In other countries, influenza activity is starting to increase but has not yet reached the epidemic threshold,” the document said.
Authorities emphasize the importance of vaccination and that countries maintain epidemiological surveillance to monitor virus activity. Below see everything we know about influenza K.
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The influenza virus, which causes the flu, is divided into four types: A, B, C, and D. Types A and B are those that circulate and cause seasonal outbreaks of the disease, usually throughout the winter. Both also have subdivisions.
Type B, for example, is separated into two lineages, B/Yamagata and B/Victoria. Influenza A has a series of subtypes classified according to the combinations of H and N proteins present on the surface of the virus.
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According to the WHO, the subtypes of influenza A currently circulating on the planet are H1N1 and H3N2. However, there are also different variants of these viruses, called subclades, depending on their genetic makeup.
This is where “flu K,” a subclade of the H3N2 virus officially called J.2.4.1, comes in. These variations are expected and are part of the evolutionary process of influenza. Hence the need to update the composition of vaccines every year.
Where does influenza K circulate?
According to the WHO alert, based on data from GISAID, a platform where researchers around the world record identifications of different viruses, “there has been a recent and rapid increase” in influenza K in different parts of the planet.
“Detections of subclade K viruses are increasing in many parts of the world, with the exception, so far, of South America. Subclade K viruses have been particularly evident from August 2025 in Australia and New Zealand and have now been detected in more than 34 countries in the last 6 months,” it says.
In a press release, the Pan American Health Organization (PAHO) indicates that, according to the most recent data, the subtype “has developed rapidly in Europe and several Asian countries, where it represents a significant proportion of the influenza A(H3N2) viruses analyzed”.
“In North America, the United States and Canada also record a gradual increase in the detection of subclade K. To date, no similar circulation has been observed in South America,” he continues.
To date, available information does not indicate that influenza K is more serious than the common illness. According to PAHO, health authorities in countries where the subtype of the virus is circulating “have not reported significant changes in the clinical severity” of the disease.
What are the symptoms of influenza K?
According to health authorities, various symptoms specifically associated with the K version of the influenza virus have not yet been identified. For now, the symptoms remain the same as those of a common infection. According to the Ministry of Health, these are:
- Fever
- Sore throat
- Faintness
- Nasal secretion
- Body pain
- Cough
- Fatigue
Do vaccines protect against influenza K?
Although data on the degree of vaccine effectiveness specifically against influenza K are still limited, initial estimates indicate that vaccination continues to protect against severe forms of the disease, particularly in the most vulnerable individuals.
“Even though there are some genetic differences between circulating influenza viruses and the strains included in vaccines, the seasonal influenza dose may still provide protection (…). Vaccination is still expected to protect against serious illness and remains one of the most effective public health measures,” says the WHO.
According to the organization, the current composition of the vaccine indicates that it is 70 to 75% effective in preventing hospital care in children aged 2 to 17 years and 30 to 40% in adults.
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