This study investigates the early alterations in the receptor-binding properties of avian influenza virus hemagglutinin (HA) after interspecies transmission to mammals, including humans and pigs. The researchers compared the HA of initial isolates from the 1957 and 1968 human pandemics, the 1979 European swine epizootic, and the 1992 seal epizootic with those of closely related avian viruses. They found that the earliest available human and swine epidemic viruses bound to 6′-sialyl(N-acetylactosamine) (6′SLN-PAA) with higher affinity compared to avian viruses, which primarily bound to 3′-sialyllactose (3′SL-PAA). For H2 and H3 strains, a single mutation (226Q→L) increased binding to 6′SLN-PAA, while for H1 swine viruses, mutations at positions 190E→D and 225G→E were crucial for increased affinity to 6′SLN-PAA. These findings suggest that alterations in the receptor-binding specificity of HA occur early after interspecies transmission and may be a prerequisite for the effective replication and spread of epidemic strains. The study also highlights the importance of specific amino acid substitutions in the HA receptor-binding site for the adaptation of avian viruses to new hosts.This study investigates the early alterations in the receptor-binding properties of avian influenza virus hemagglutinin (HA) after interspecies transmission to mammals, including humans and pigs. The researchers compared the HA of initial isolates from the 1957 and 1968 human pandemics, the 1979 European swine epizootic, and the 1992 seal epizootic with those of closely related avian viruses. They found that the earliest available human and swine epidemic viruses bound to 6′-sialyl(N-acetylactosamine) (6′SLN-PAA) with higher affinity compared to avian viruses, which primarily bound to 3′-sialyllactose (3′SL-PAA). For H2 and H3 strains, a single mutation (226Q→L) increased binding to 6′SLN-PAA, while for H1 swine viruses, mutations at positions 190E→D and 225G→E were crucial for increased affinity to 6′SLN-PAA. These findings suggest that alterations in the receptor-binding specificity of HA occur early after interspecies transmission and may be a prerequisite for the effective replication and spread of epidemic strains. The study also highlights the importance of specific amino acid substitutions in the HA receptor-binding site for the adaptation of avian viruses to new hosts.