The NCIPD analyzed 23 clinical samples with influenza A viruses, subtypes A(H3N2) (20 samples) and A(H1N1) (3 samples), using next-generation whole-genome sequencing.
The samples were taken from patients exhibiting symptoms of influenza or acute respiratory infections, aged 2 months to 54 years, who were treated in 7 regions of the country between 24.11 and 17.12.2025. The Regional Health Inspectorate and Multi-profile Hospitals sent the samples.
The NCIPD analyzed 23 clinical samples with influenza A viruses, subtypes A(H3N2) (20 samples) and A(H1N1) (3 samples), using next-generation whole-genome sequencing.
The samples were taken from patients exhibiting symptoms of influenza or acute respiratory infections, aged 2 months to 54 years, who were treated in 7 regions of the country between 24.11 and 17.12.2025. The Regional Health Inspectorate and Multi-profile Hospitals sent the samples.
It was found that all sequenced samples of subtype A(H1N1) belong to clade 5a.2a.1, subclade D.3.1.1. This subclade is new for the country and was not detected in the previous season. In the sequenced A(H3N2) samples, the “new” subclade K (clade 2a.3a.1) was identified in 95% of the samples, while subclade J.2.4 (clade 2a.3a.1) was also demonstrated in one of the samples. Subclade K (formerly J.2.4.1) became the dominant circulating A(H3N2) strain in many parts of the world, including the EU/EEA and North America, during the 2025-2026 influenza season.
The final analysis to determine the viral variants was performed using the Nextclade v.3.18.1 program.
Vaccine match: The 2025-2026 Northern Hemisphere influenza A(H3N2) vaccine strain A/Croatia/10136RV/2023 showed low reactivity with novel subclade K viruses in laboratory tests, suggesting potential mismatch.1
Vaccine effectiveness: Despite potential mismatch, health authorities such as the ECDC and the UK HSA emphasize that the current vaccine is expected to protect against severe illness, hospitalization, and death, making it a vital public health tool.2
1 Kirsebom, F. C., Thompson, C., Talts, T., Kele, B., Whitaker, H. J., Andrews, N., et al. (2025). Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K. Euro surveillance, 30(46), 2500854. https://doi.org/10.2807/1560-7917.ES.2025.30.46.2500854
2 https://www.ecdc.europa.eu/sites/default/files/documents/influenza-risk-assessment-EU-EEA-circulation-AH3N2-subclade-K.pdf