Objective To analyze the hemagglutinin variation and the genetic evolution of A(H1N1)pdm09 influenza virus in Huzhou City, 2023.

Methods Respiratory tract specimens from two influenza monitoring hospitals were collected for the isolation of A(H1N1)pdm09 influenza virus and HA genome sequencing. MEGA 7.0 and BioAider were utilized to analyze genetic similarity, genetic evolution, antigen mutation sites, receptor binding sites, pathogenicity, and potential glycosylation sites of the influenza virus sequencing results.

Results The nucleotide and amino acid similarities between the 25 A(H1N1)pdm09 strains and the vaccine strain A/Wisconsin/67/2022 were 98.41% to 99.22% and 98.41% to 99.36%, respectively. These strains belonged to the 6B.1A.5a.2a evolutionary lineage. Notably, there were 19 amino acid variations in the HA protein, including substitutions of S137P and R142K in the epitope Ca2 cluster. Additionally, seven potential glycosylation sites were found in the HA protein, and no new or missing sites were observed in the 25 A(H1N1)pdm09 influenza virus isolates compared to the vaccine strains. Furthermore, no multiple continuous alkaline amino acids were detected in the HA cleavage site of the 25 isolates.

Conclusion The A(H1N1)pdm09 pandemic strain in 2023 closely matches the vaccine strain, indicating good protection against the currently circulating A(H1N1)pdm09 influenza virus. The circulating A(H1N1)pdm09 strain in Huzhou is characterized as a low-pathogenic influenza virus.