In the 2025–2026 influenza season, antigenic drift is occurring, which is the natural evolution of the influenza virus caused by minor genetic changes in its surface proteins[1][2][4][9]. The dominant variant is A(H3N2) subclade K or sublineage J.2.4.1, which accounts for up to 90% of confirmed cases in Europe and spreads more rapidly due to mutations that allow evasion of the immune response[3][5][6][7]. The season started 3–4 weeks earlier than usual, which increases the pressure on health systems[3][7]. For this season, trivalent vaccines protecting against strains A(H1N1), A(H3N2) and B/Victoria are being switched, replacing the previous quadrivalent[2][4]. The effectiveness of vaccines against hospitalizations is around 35% in risk groups, in better years over 50%[1][4]. Vaccination is recommended from 6 months of age to risk groups such as the elderly, pregnant women, young children and patients with chronic diseases, ideally in October to December[2]. Vaccines stimulate the immune system to produce antibodies without causing disease[2].