Introduction: Genomic and phenotypic surveillance of the Influenza A(H3N2) virus is essential to monitor viral evolution and support decisions related to vaccine composition.Objectives: This study evaluated mutations in the antigenic sites A-E of the hemagglutinin (HA) of A(H3N2) viruses detected in Brazil between January 2025 and January 2026. Methodology:Complete HA sequences of the Influenza A(H3N2) virus and the three vaccine strains recommended by the WHO during the study period were obtained from the EpiFlu database (GISAID).The sequences were aligned and converted into amino acids to evaluate localized changes in the A-E antigenic sites, with emphasis on the A and B sites, which are historically associated with a greater antigenic impact. Results:The results revealed 1,620 A(H3N2) HA sequences recovered from Brazil, detecting 6 subclades (J.2, J.2.2, J.2.3, J.2.4, J.2.5, K) with multiple relevant substitutions in the antigenic sites.The transition from subclade J.2 to J.2.3 (EW12 to EW23) was observed, followed by the rapid emergence of subclade K (EW44/2025 to EW04/2026).We identified the fixation of mutations in antigenic site A (T135K, S144N, and N145S) and site B (N158D, I160K, and Q173R) within subclade K, some of these alterations (S144N, N158D, I160K, and Q173R) are not present in the A/Singapore/GP20238/2024 vaccine strain chosen for the 2026 Southern Hemisphere vaccine, however, the A/Darwin/1454/2025 strain selected for the Northern Hemisphere 2026/2027 season already includes these substitutions.This suggests a potential antigenic drift and reinforces the need for phenotypic testing to better characterize the circulating viruses.Additional alterations in sites C, D, and E were documented, although less frequently in isolated samples, contributing to the structural diversity of the glycoprotein. Conclusion:The comparative analysis indicates that the observed genetic diversity may influence vaccine effectiveness, requiring confirmation through antigenic analyses.This work reinforces the importance of integrated molecular surveillance to guide public health responses and future vaccine update.
Ramos et al. (Thu,) studied this question.