Genetic Diversity and Biological Characteristics of H3 Avian Influenza Virus Isolated from China in 2021-2022 Showed the Emerging H3N8 Posed a Threat to Human Health

The H3 influenza viruses are widespread in domestic poultry but have been ignored because their pathogenicity in poultry is low. Three human infections with H3N8 influenza viruses have been reported in China since 2022, raising public concern. Here, we comprehensively analyzed 30 H3 subtype avian influenza viruses isolated from live poultry markets in China between 2021 and 2022. Genetic and phylogenetic analyses showed that the H3 viruses have undergone frequent reassortment and have formed complex genotypes. Notably, the viruses that caused human infections in 2022-2023 were highly homologous to the H3N8 viruses circulating in poultry in 2022, with internal genes derived from the H9N2 viruses. The analysis of chicken infections indicated that the novel H3N8 viruses were more infectious in chickens than those that do not carry H9N2 genes, whereas the H3 viruses detected in China in 2021-2022 showed low pathogenicity in mice. Our findings suggest that the novel H3N8 viruses bearing internal H9N2 genes have adapted to and circulated in chickens and pose a threat to human health. These results highlight the need for continued surveillance of the H3 influenza viruses and their impact on the poultry industry.