Objective To understand the viral epidemiology of influenza-like illness (ILI) and the genetic evolutionary characteristics of influenza B virus in Mianyang, Sichuan province.
Methods The pharyngeal swabs from ILI cases were collected in Mianyang from 2021 to 2022, and 16 viral pathogens were detected by using real-time fluorescence quantitative PCR. Influenza B virus was isolated by using Madin-Daby canine kidney (MDCK) cells from positive samples. The hemagglutinin (HA) and neuraminidase (NA) genes of isolated strains were sequenced and analyzed for genetic evolution, homology and amino acid site variation.
Results A total of 332 samples of ILI cases were collected in Mianyang from 2021 to 2022, in which 195 were virus positive (58.73%, 195/332). In the virus positive samples, 114 were influenza B virus positive (34.34%, 114/332), the other samples were human coronavirus-HKU1 positive (7.53%), human rhinovirus positive (6.93%) and respiratory syncytial virus positive (6.63%). A total of 40 strains of influenza B virus were successfully isolated. The HA and NA gene amplification and sequencing of the virus showed that all the 40 influenza B virus strains belonged to Victoria lineage, belonging to evolutionary branches of V1A.3a.1 and V1A.3a.2. The two branches of the virus showed genetic recombination. The HA and NA genes shared 99.22%-99.81% and 98.64%-99.77% homologies with the reference vaccine strain Washington/02/2019, respectively. Compared with vaccine strain, several amino acid locus mutations were observed in both HA and NA genes of the 40 strains, but no drug resistance loci were found.
Conclusion Multiple viruses can be detected in the samples of ILI cases in Mianyang from 2021 to 2022. Influenza B was predominant, mainly belonging to evolutionary branches V1A.3a.1 and V1A.3a.2, and multi-amino acids mutations of HA and NA proteins of the virus occurred. It is necessary to further strengthen the surveillance for respiratory virus infection in Mianyang.
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