Genomic evolution, transmission dynamics, and pathogenicity of avian influenza A (H5N8) viruses emerging in China, 2020.

Multiple recent outbreaks of highly pathogenic H5N8 viruses originating in aquatic birds frequently occurred in most European countries, Russia, South Korea, and Japan during the winter of 2020-21, and one zoonotic event of poultry workers infected with novel H5N8 viruses were reported in Russia. Strikingly, these novel H5N8 viruses had emerged and been co-circulating in wild birds and poultry in multiple provinces of China during 2020-21. In China, the population of aquatic birds has risen significantly in the past twenty years, and China is regarded as the largest reservoir for influenza viruses carried in aquatic birds across the globe. Hence, the co-circulation of these novel H5N8 viruses poses an alarming threat to not only poultry industry but also human health. In this study, we sequenced full-length genomes of these H5N8 viruses circulating in China. Phylogenetic analysis demonstrated that poultry-origin H5N8 viruses in China fell within wild birds-origin clade 2.3.4.4b H5N8 viruses from Europe during 2020-21, and notably, were genetically closely related to human-infecting H5N8 viruses in Russia. Moreover, they possessed several molecular markers associated with mammalian adaption. Bayesian coalescent analysis showed that these H5N8 viruses might have introduced into China during June-September 2020, suggesting that these H5N8 viruses might have introduced via wild bird migration or poultry trade. Besides, we also found that the effective population size of clade 2.3.4.4b H5N8 viruses dramatically increased during the winter season of 2020/21, as is consistent with previous increase of genetic diversity during the winter seasons of 2013/14 and 2016/17, which indicated that the wild bird migration accelerates the genetic diversity of these H5N8 viruses during the winter season of 2020/21. Notably, these novel H5N8 viruses were lethal to chickens and mice, highly transmissible to ducks, and were antigenically distinct from 2.3.4.4h H5 viruses circulating in China, posing considerable threats to public health. Our findings offer novel insights into the evolution and risk assessment of H5N8 viruses during the winter season of 2020-21.