Rapid expansion of genotype D1.1 A(H5N1) influenza viruses in wild birds across North America during the 2024 migratory season.

In late 2021, high pathogenicity avian influenza A(H5N1) clade 2.3.4.4b viruses entered North America and reassorted rapidly with local avian influenza viruses. In September 2024, we detected a new reassortant later classified as genotype D1.1. Using active and passive avian influenza surveillance across Canada and the USA, we tracked the emergence and rapid spread of D1.1 viruses in wild birds during the 2024 fall migration. Phylodynamic analysis showed that D1.1 viruses formed a monophyletic group and displaced earlier A(H5) genotypes across several flyways. Their expansion coincided with detections in other hosts, including 17 human cases, 4 of which were severe or fatal. None of the mammalian-adaptive markers detected in human cases were found in wild bird viruses, and candidate vaccine viruses retained antigenic cross-reactivity with D1.1 strains.