Molecular evolution and characterization of hemagglutinin and neuraminidase of influenza A(H1N1)pdm09 viruses isolated in Beijing, China, during the 2017-2018 and 2018-2019 influenza seasons

We investigated and analysed the molecular evolution of hemagglutinin (HA) and neuraminidase (NA) of influenza A(H1N1)pdm09 virus during the 2017-2018 and 2018-2019 influenza seasons in Beijing, China. We collected and extracted RNA from influenza A(H1N1)pdm09 strains from Peking University People's Hospital and analyzed their HA and NA genes by RT-PCR and sequencing. Phylogenetic analysis of HA and NA sequences was used to compare the amino acid sequences of 51 strains with those of reference strains. All strains belonged to subclade 6B.1, with S162N and I216T substitutions (H1 numbering). Our strains differed from strain A/Michigan/45/2015, with the substitutions S91R, S181T and I312V in the HA antigenic epitope. An E189G mutation was detected in the 190 helix of the receptor binding region of HA. A new potential glycosylation site, 179 (NQT), which was not detected before the 2015 influenza season, was identified. Two strains were mutated at I223, the NA inhibitor resistance site. During 2012-2019, amino acids of HA and NA mutated over time. Co-occurrence mutations N146D, S200P, S202I and A273T in HA appeared along with Q51K, F74S and D416N in NA in six strains during two influenza seasons. Our work reveals the molecular changes and phylogenetic characteristics of influenza A(H1N1)pdm09 virus and suggests that a vaccine probably provides suboptimal protection. The biological characteristics of the new glycosylation and drug-resistance sites detected in this work need to be studied further. The co-occurrence of mutations in HA and NA might affect the characteristics of the virus and need to be given more attention.