Genetic and antigenic characterization of influenza A(H1N1)pdm09 in Yantai, China, during the 2009-2017 influenza season.

Objective This study was performed to determine the antigenic and genetic characteristics and evaluate potential vaccine efficacy of influenza A (H1N1)pdm09 in Yantai from August 2009 to August 2017. Materials and Methods A total of 10 236 swabs were collected among patients with an influenza-like illness who were admitted to two sentinel surveillance hospitals in Yantai, East China, from August 2009 to August 2017. All specimens were cultured in Madin-Darby canine kidney cells and identified by haemagglutination-inhibition assay. Complete sequences of haemagglutinin (HA) and neuraminidase of 51 influenza A(H1N1)pdm09 strains circulating in Yantai were amplified, sequenced and analysed using molecular and phylogenetic methods. The potential vaccine efficacy was calculated using the p(epitope) model which measured the antigenic variation based on the changes in the dominant epitope of HA. Results The results showed that most Yantai strains were grouped into genetic clades 1.7, 6C, 6B.1 and 6B.2. The amino acid substitutions accumulated gradually in HA proteins and considerable genetic variation were observed in circulating A(H1N1)pdm09 viruses during the seven influenza seasons. The V241I, N369K, N386K and K432E mutations which may change the binding pattern and affinity of oseltamivir for neuraminidase were detected in the strains circulating in 2016/2017 and 2017/2018 seasons and the recommended vaccine strains could afford optimal protection against the influenza A/H1N1pdm09. Conclusions Although influenza A(H1N1)pdm09 viruses acquired significant genetic variation over the course of seven influenza seasons, the recommended vaccine strains still afforded protection against main circulating strains. Continuous epidemiological and virological surveillance are necessary.