Epidemiological characteristics of seasonal influenza viruses and molecular evolution analysis of influenza B virus in Shenyang, Liaoning province, 2018−2022

Objective To understand the epidemiological characteristics of influenza viruses in Shenyang, Liaoning province, by studying the pathogenetic surveillance of influenza viruses in the monitoring cycle from 2018−2022, and also to analyze the molecular features and genealogical evolutionary characteristics of influenza virus epidemics in Shenyang by amplifying the Hemagglutinin full-length genes of influenza B viruses.

Methods  Throat swab samples from influenza-like illness in out-patient clinics of Shenyang Sentinel Hospital from October 1, 2018 to March 31, 2022 were collected. After confirming positive nucleic acid of influenza virus by quantitative real-time polymerase chain reaction (qPCR), the virus was isolated. Some influenza B strains were selected and their full-length heamagglutinin (HA) gene was amplified and sequenced by reverse transcription polymerase chain reaction (RT-PCR). Snapgene software was used to spline the sequencing results, ClustalW was used to compare multiple sequences, and MEGA7.0 software was used to construct an evolutionary tree to analyze amino acid mutation sites.

Results  The total positive rate of influenza viruses detected in Shenyang in the 2018−2022 surveillance cycle was 5.00%, which was mainly dominated by A/H1N1 viruses (44.38%), followed by A/H3N2 viruses (28.37%) and B/Victoria (27.24%), and no B/Yamagata lineage strains were detected. However, B/Victoria strains accounted for 83.33% and 86.66% as the absolute dominant strains in the epidemic seasons of 2020−2021 and 2021−2022, respectively. And the age group of 5–14 years old was susceptible in the population distribution. The Hemagglutinin genes of the B/Victoria strains isolated in Shenyang were all on the V1A branch. Compared with the vaccine strain, the epidemic strain had multiple amino acid site mutations in key antigen determinant sites of Hemagglutinin protein, such as 120-loop, 150-loop, 160-loop and 190-helix.

Conclusion  In the 2018−2022 surveillance cycle , the influenza virus subtype diversity decreased, but the B/Victoria strain genetic diversity increased, and mutations occurred in several key antigenic determinant. Therefore, the epidemiological characteristics and molecular evolution of influenza need to be continuously monitored. To provide scientific basis for the prevention and control of respiratory infectious diseases mainly influenza.