Influenza virus (FLU) is a major cause of respiratory tract infections and remains a substantial public health burden. It is transmitted via respiratory droplets and primarily affects the upper respiratory tract. In humans, influenza virus A (FluA) and influenza virus B (FluB) are the most epidemiologically relevant types. The prevalence of different FLU types and antigenic variants, as well as the presence of genotypic resistance to neuraminidase inhibitors, were investigated. Possible associations between antiviral resistance and age, immune status, specific antigenic virus variants, and antiviral treatment were evaluated. Archived respiratory samples from patients with laboratory-confirmed FluA infection, submitted to the Institute of Microbiology and Immunology, Faculty of Medicine, University of Ljubljana, were included in the study. The H275Y mutation was detected by amplification of 183 bp fragment of the neuraminidase gene, followed by allele discrimination analysis using real-time RT-PCR. For subtype A(H1N1), 621 bp and 358 bp fragments of the neuraminidase gene were amplified, while a 743 bp fragment was amplified for subtype A(H3N2). Nucleotide sequences were determined using a genetic analyzer. Between 2022 and 2025, alternating seasonal circulation of subtypes A(H1N1) and A(H3N2) was confirmed, whereas FluB occurred sporadically. The H275Y mutation was detected in a small proportion of A(H1N1) samples, while no clinically relevant resistance-associated mutations were identified in A(H3N2). The H275Y mutation was more frequently observed in older patients and in those treated with oseltamivir; however, no statistically significant association with immune status or treatment was demonstrated. Overall, a low prevalence of genotypic resistance with limited circulation of resistant viral variants in the population was identified.
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