Researchers sequenced clade 2.3.4.4b H5N1, H5N6 and H5N9 viruses from 62 poultry outbreaks and 63 wild bird cases in Korea over winter 2025-26, and tested two isolates in chickens and ducks. Most genotypes were reassortants of East Asian H5N1 with low-pathogenicity wild bird viruses, and the European genotype EA-2024-DI.2.1 was also detected. H5N1 and H5N9 were highly infective in chickens (median lethal doses of 10^3.3 and 10^3.2 EID50); in young ducks H5N1 killed rapidly while H5N9 caused no mortality but still transmitted to all contact birds.
Why it is interesting: A European H5N1 genotype reached East Asia within a season and reassorted locally, with H5N9 transmitting silently in ducks.