Genetic characterization of influenza A(H1N1)pdm09 virus in 2023 in Huzhou, China

Introduction: Since its identification in April 2009, influenza A(H1N1)pdm09 virus has continued to cause significant outbreaks of respiratory tract infections, including pandemics in humans. In the course of its evolution, the virus has acquired many mutations that cause increased disease severity. Regular molecular surveillance of the virus is essential to mark the evolutionary changes that may cause a shift in viral behavior. Methods: Twenty-five (H1N1)pdm09 isolates from Huzhou, China, in 2023 were randomly selected for wholegenome sequencing along with analysis of nucleotide and amino acid sequence identity, average genetic distance, antigen variation sites, receptor binding sites, pathogenicity, potential glycosylation sites, and drug resistance gene sites. Results: The 25 (H1N1)pdm09 strains and representative 2023-2024 northern hemisphere vaccine strain A/ Wisconsin/67/2022 showed nucleotide and amino acid similarity of 98.41 %-99.22 % and 98.41 %-99.36 %, respectively. This virus belongs to the 6B1A.5a.2a evolutionary lineage. There were 19 amino acid variations in hemagglutinin (HA) protein. Substitutions S137P and R142K occurred in the epitope Ca2 cluster. A total of 13 amino acids were mutated in neuraminidase (NA) protein, and 9 key protease active sites and amino acid sites related to NA inhibitor (NAI) resistance were not mutated. Five amino acid sites of matrix-2 (M2) protein associated with drug resistance were not replaced, but amino acid 31 of M2 protein was shown to be N in the 2023 pandemic strain (H1N1)pdm09 and vaccine strain A/Wisconsin/67/2022. There were seven and eight potential glycosylation sites for HA protein and NA protein, respectively. Compared with the vaccine strains, there were no new or missing potential glycosylation sites in the 25 (H1N1)pdm09 influenza virus isolates. No multiple continuous alkaline amino acids were found in the HA cleavage site of the 25 isolates. Conclusions: The recommended vaccine strain A/Wisconsin/67/2022 has good prevention potential against the current circulating influenza (H1N1)pdm09. At present, NAIs have good therapeutic and preventive effects against (H1N1)pdm09 circulating in Huzhou, China, but the virus is naturally resistant to amantadines. At present, the (H1N1)pdm09 strain circulating in Huzhou, China, is characterized by low pathogenicity. However, regular molecular surveillance of (H1N1)pdm09 is important to monitor the behavior of the virus in terms of increases in virulence, drug resistance, and emergence of novel strains.