2025–2026 seasonal flu: what we need to know about the Influenza A(H3N2) subclade K strain
The 2025–2026 flu season began earlier in many regions of Europe and is characterized by the predominant circulation of the Influenza A(H3N2) virus, specifically an emerging sub-clade known in the literature as sub-clade K. Data provided by the World Health Organization (WHO) and the European Centre for Disease Prevention and Control (ECDC) confirm sustained transmission of this viral subtype, associated with a significant impact on healthcare systems [1][2].
Table of Contents
1. What is the flu and what viruses cause it
2. Influenza A(H3N2) sub-clade K – what we know from official data
3. Flu transmission and the period of contagiousness
4. High-risk individuals
5. Flu symptoms
6. Flu complications
7. Diagnosis
8. Treatment
9. Prevention – an essential pillar in the flu season
1. What is the flu and what viruses cause it
The flu is an acute, highly contagious respiratory infection caused by influenza viruses of the Influenza A and B genera. Influenza A viruses are responsible for the majority of seasonal epidemics and can undergo frequent genetic changes (antigenic drift), which leads to the emergence of new variants and sub-clades [3].
The Influenza A(H3N2) subtype is known for its high antigenic variability and for its more severe impact on vulnerable populations, especially the elderly and individuals with chronic diseases [3][4].
2. Influenza A(H3N2) sub-clade K – what we know from official data
Genomic sequencing analyses conducted within the global influenza surveillance network (WHO GISRS) identified, in 2025, an increase in the circulation of a new sub-clade of the A(H3N2) virus, named sub-clade K [1].
According to the ECDC:
∙ sub-clade K shows antigenic changes in the hemagglutinin;
∙ these changes may partially reduce recognition by antibodies generated by seasonal vaccines;
∙ to date, there is no clear evidence that sub-clade K causes greater clinical severity compared to the common seasonal H3N2 virus; however, the number of cases may be higher in populations with reduced immunity [2].
3. Flu transmission and the period of contagiousness
The influenza virus is transmitted:
∙ through respiratory droplets expelled by coughing, sneezing, or talking;
∙ through direct contact with contaminated surfaces and then with mucous membranes (nose, mouth, eyes) [5].
Classic studies show that influenza viruses can survive from a few minutes to 24–48 hours on various surfaces, depending on the material and environmental conditions [6].
An infected person is contagious:
∙ 1–2 days before the onset of symptoms;
∙ up to 5–7 days after the onset of the illness (longer in children and immunocompromised individuals) [5].
4. High-risk individuals
According to the WHO and CDC, the risk of flu complications is higher in [4][7]:
∙ children under 5 years old (especially under 2 years old);
∙ adults over 65 years old;
∙ pregnant women;
∙ individuals with chronic cardiovascular, respiratory, metabolic, renal, neurological, or oncological diseases;
∙ immunocompromised individuals;
∙ unvaccinated individuals.
5. Flu symptoms
The flu usually has a sudden onset, with the following manifestations [7][8]:
∙ high fever (38–40°C);
∙ chills;
∙ muscle and joint pain;
∙ headache;
∙ dry cough, sore throat;
∙ marked fatigue;
∙ loss of appetite.
In young children, digestive symptoms (vomiting, diarrhea) may occur, although these are not typical manifestations in adults [7].
6. Flu complications
If left untreated or in vulnerable individuals, the flu can lead to severe complications, such as [9]:
∙ viral or bacterial pneumonia;
∙ exacerbation of chronic diseases;
∙ respiratory failure;
∙ myocarditis, encephalitis;
∙ severe dehydration.
7. Diagnosis
∙ rapid antigen tests for Influenza A/B;
∙ RT-PCR for identifying the H3N2 subtype;
∙ genomic sequencing for epidemiological surveillance purposes.
8. Treatment
Flu treatment is primarily symptomatic. Antivirals (such as oseltamivir) may be recommended by a doctor, ideally within the first 48 hours of onset, especially in high-risk patients [11].
Antibiotics are not indicated unless bacterial superinfections are confirmed.
9. Prevention – an essential pillar in the flu season
Flu vaccination
Annual vaccination remains the most effective method of prevention, reducing the risk of severe forms and hospitalization, even when antigenic matching is not complete [1][2].
Hand and surface hygiene
Rigorous hand and surface hygiene, including the use of disinfectants with proven virucidal spectrum, is essential to limit the transmission of the virus through direct or indirect contact [5][6].
Other measures
Other preventive measures include ventilating spaces, avoiding contact with symptomatic people, wearing a mask in crowded spaces during peak epidemic periods, and isolating symptomatic patients.
The 2025–2026 flu season is marked by the predominant circulation of the Influenza A(H3N2) sub-clade K virus. WHO and ECDC data indicate sustained transmission, without clear evidence of increased individual severity, but with a potential major impact at the population level. Vaccination, rigorous hygiene, and the correct use of disinfectants remain key elements to limit the spread of the flu virus and protect public health. The information provided is for informational purposes only and does not replace medical consultation.
Bibliography
1. World Health Organization (WHO). Seasonal influenza – Global situation update, 2025. Geneva: WHO, 2025. https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON586
2. European Centre for Disease Prevention and Control (ECDC). Risk assessment on the circulation of Influenza A(H3N2) sub-clade K in the EU/EEA. Stockholm: ECDC, 2025.
https://www.ecdc.europa.eu/sites/default/files/documents/influenza-risk-assessment-EU-EEA-circulation-AH3N2-subclade-K.pdf
3. World Health Organization (WHO). Influenza (Seasonal) Fact Sheet. Geneva: WHO, 2024.
https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)
4. Centers for Disease Control and Prevention (CDC). People at Higher Risk of Flu Complications. Atlanta: CDC, 2024. https://www.cdc.gov/flu/highrisk/index.htm
5. Centers for Disease Control and Prevention (CDC). How Flu Spreads. Atlanta: CDC, 2024.
https://www.cdc.gov/flu/spread/index.html
6. Bean, B., Moore, B. M., Sterner, B., Peterson, L. R., Gerding, D. N., & Balfour, H. H. Jr. Survival of Influenza viruses on environmental surfaces. Journal of Infectious Diseases, 146(1), 47–51, 1982. https://pubmed.ncbi.nlm.nih.gov/6282993/
7. Centers for Disease Control and Prevention (CDC). Flu Symptoms & Diagnosis. Atlanta: CDC, 2024. https://www.cdc.gov/flu/signs-symptoms/index.html
8. Mayo Clinic Staff. Influenza (flu): Symptoms and causes. Rochester, MN: Mayo Clinic, 2024. https://www.mayoclinic.org/diseases-conditions/flu/symptoms-causes/syc-20351719
9. Centers for Disease Control and Prevention (CDC). Flu Complications. Atlanta: CDC, 2016. https://www.cdc.gov/flu/about/disease/complications.htm
10. Cleveland Clinic. Flu (Influenza) Tests. Cleveland, OH: Cleveland Clinic, 2022.
https://my.clevelandclinic.org/health/diagnostics/22716-flu-influenza-test
11. World Health Organization (WHO). Clinical practice guidelines for influenza. Geneva: WHO,
2024. https://www.who.int/publications/i/item/9789240097759
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