This receptor specificity has implications for human infection with avian influenza virus strains. For example, highly pathogenic avian H5N1 influenza viruses undergo limited replication in the human respiratory tract due to the presence of some cells with alpha(2,3) linked sialic acids. However, efficient human to human transmission requires that the avian viruses recognize sialic acids with alpha(2,6) linkages. Consistent with this hypothesis, the results of studies of early influenza virus isolates from the 1918, 1957, and 1968 pandemics suggest that these viruses preferentially recognized alpha(2,6) linked sialic acids.