Abstract:This study aimed to understand the molecular characteristics and explore specific molecular targets of BV lineage strains involved in influenza epidemics and severe pneumonia cases in Guizhou province, to provide scientific evidence for influenza prevention and control. The BV lineage strains involved in influenza outbreaks, severe cases and common epidemics in Guizhou Province from 2017 to 2019 were examined. After genomic extraction, HA and NA gene amplification, and sequencing, the molecular characteristics of these strains together with reference strains were analyzed. The topological structure of the HA and NA evolutionary trees of all strains in Guizhou province was similar, and the strains formed three branches. In 2017, the strains were clustered, whereas in 2018, they were scattered. In 2019, they formed two relatively distant branches. Most of the outbreak strains and severe case strains were in branch 1. The sequence identity of HA and NA genes of all strains in Guizhou province was highest, at 98.6%-993% and 98.9%-99.4%, respectively. The sequence identity of HA and NA genes among BV lineage strains was 99.9% and 99.9%-100%, respectively, in 2017; 98.4%-99.3% and 98.7%-99.1%, respectively, in 2018; and 97.7%-100% and 98.4%-99.9%, respectively, in 2019. In 2019, the sequence identity of the strains in branch 2 was 99.4%-100% and 99.6%-99.9%, respectively, and that in branch 1 was 995%-100% and 99.3%-99.9%, respectively. The sequence identity between the strains involved in all outbreaks and severe cases and the common epidemic strains in the current year was higher than 99.4%. Compared with those in vaccine strain B/Colorado/06/2017, all HA genes of the strains had mutations of S14N, G144D, V195I and K513R, and all NA genes had mutations of Q371K and D490N. The mutations G148R, K151E and T563A were found in the HA genes, and A395T was found in the NA genes of the strains in branch 1; V394I in HA genes and V71A, K343E, A395V and V401I in NA genes were found in the strains in branch 2. The HA genes of the strains in branch 1 had a 179-181KND deletion mutation, as compared with B/Brisbane/60/2008, and an 181D deletion mutation and 150 aera antigen locus mutation, as compared with B/Colorado/06/2017. In conclusion, a gap was observed among BV lineage strains in Guizhou province, but the genetic relationships and sequence identities were strong between BV lineage strains and common epidemic strains. Two cladistic strains were found in 2019, and the strains in branch 1 were the main strains involved in outbreaks and severe pneumonia cases. Further strengthening of real-time monitoring of molecular characteristics and further study of specific molecular targets related to the pathogenicity of BV lineage influenza virus are needed.