Genetic characteristics of influenza A/H3N2 virus in Jiaxing, China (2019–2024)

The rapid evolution of influenza A/H3N2 virus underscores the imperative for ongoing surveillance. In this study, we analyzed 7,872 respiratory samples collected from patients with influenza-like symptoms in Jiaxing, China, between 2019 and 2024, selecting 117 influenza A/H3N2 strains for whole-genome sequencing. Between 2019 and 2024, the influenza A/H3N2 virus in Jiaxing experienced six epidemic peaks, evolving from clades 3 C.2a1b.1a/1b, 3 C.2a1b.2, and 3 C.2b1b.2a.2a.3/3b to clade 3 C.2a1b.2a.2a.3a.1. Compared with the 2019-2021 period, influenza A/H3N2 viruses circulating in Jiaxing during 2022-2024 demonstrated a longer evolutionary distance and increased hemagglutination titers. Additionally, seven novel amino acid substitutions (H156S, Y159N, T160I, S186D, D190N, I192F, N96S) were identified in HA antigenic epitopes of circulating influenza A/H3N2 viruses during 2023-2024. Notably, based on predicted vaccine effectiveness, the 2023-2024 influenza vaccine retained strong protective efficacy. In conclusion, this study characterizes the evolutionary features of influenza A/H3N2 virus in Jiaxing during 2019-2024. These results enhance our understanding of the natural evolution of influenza virus and the emergence of new variants, providing critical insights into influenza vaccine effectiveness.