Since 2014, highly pathogenic avian influenza H5N6 viruses have been
responsible for outbreaks in poultry. In this study, four H5N6 virus
strains were isolated from fecal samples of sick white ducks and dead
chickens in Shandong in 2019. These H5N6 viruses were triple-reassortant
viruses that have not been previously characterized. Their HA genes were
derived from the H5 viruses and were closely related to the vaccine
strain Re-11. Their NA genes all fell into the N6-like lineage and the
internal gene were derived from H5N1 and H9N2 viruses. They all showed
high pathogenicity in mice and caused lethal infection with high rates
of transmission in chickens. Moreover, the SPF chickens inoculated with
the current used vaccine in China were completely protected from these
four H5N6 viruses. Our study indicated the necessity of continued
surveillance for H5 IAV and the importance of timely update of vaccine
strains in poultry industry.