Objective: In the present study, we aimed to investigate the phylogenetic relationships of influenza B viruses circulating in Türkiye and neighboring countries from 2010 to 2024 using full-length hemagglutinin (HA) genes available on GISAID. Additionally, we aimed to determine the antigenic distances and evaluate the potential impact of these factors and demographic data on virus epidemiology.

Methods: Full-length HA gene sequences of Victoria and Yamagata viruses from Türkiye, Ukraine, Greece, Bulgaria, Georgia, Armenia, Iran, and Syria (2010–2024) were aligned using MAFFT. Phylogenetic relationships were analyzed using IQ-TREE with the ultrafast bootstrap method (1,000 replicates). Patients were categorized into age groups based on demographic data. Antigenic distances between seasons were calculated using the Needleman-Wunsch algorithm.

Results: Victoria viruses were predominantly detected in children and adolescents (0–18 years), while Yamagata viruses were more common in adults (19 years and older). Phylogenetic analyses revealed more pronounced genetic differences between seasons for Victoria viruses, whereas Yamagata viruses showed limited variation. This suggested that age might play a less significant role in the spread of viruses within geographically close regions. Furthermore, the antigenic sites of HA proteins in Yamagata viruses were more stable, while those in Victoria viruses showed frequent and dynamic changes.

Conclusion: The study demonstrated that changes occurring in the antigenic regions of the HA protein might play a significant role in the evolutionary dynamics and epidemiology of influenza B viruses from different lineages. Moreover, the limited geographic spread and conserved antigenic structures of Yamagata viruses between seasons may contribute to their disappearance from the population.