Abstract
The highly pathogenic avian influenza 2.3.4.4b H5 viruses have been a cause for concern recently, as they have been responsible for continuous outbreaks since 2021. In China, the H5N6 subtype has been predominantly circulating in domestic poultry but has rarely been detected in wild birds over the past 3 years. In December 2023, novel reassortant 2.3.4.4b H5N6 viruses were resurgent in wild birds and domestic ducks in Eastern Asia. The viruses were reassorted with those of currently prevalent 2.3.4.4b H5N1 viruses of wild bird origin worldwide, as well as the H5N6 viruses that caused human infections in 2022 and low pathogenic avian influenza viruses, such as the H9N2 virus, which also contributed internal gene to the novel H5N6 viruses. Based on the phylogenetic analyses, we inferred that this recombination process occurred in migratory breeding sites in early 2023. Given the rapid transmission and high mutation capacity of currently circulating H5N1 viruses, as well as the strong pathogenicity of H5N6 viruses to humans, the novel recombinant viruses may continue to evolve and pose new threats to human health. Therefore, continuous surveillance of H5N6 viruses in wild birds and domestic poultry should be strengthened.