Abstract
An increase in the number of human cases of influenza A/H5N1 infection in the USA has raised concerns about the pandemic potential of the virus. Pre-existing population immunity is a key determinant for risk assessment and pandemic potential for any virus. Antibody responses against the bovine A/H5N1 hemagglutinin (HA) and neuraminidase (NA) proteins were measured among a population of influenza-vaccinated or influenza-infected individuals. Modest titers of bovine A/H5N1 HA-binding antibodies and low to undetectable neutralizing antibody titers were detected in a cohort of 73 individuals. Conversely, bovine A/H5N1 NA-binding and neuraminidase-inhibiting antibody titers were comparable to those against a human A/H1N1 NA at baseline. Seasonal influenza vaccination failed to significantly increase antibody titers against both HA and NA glycoproteins of bovine A/H5N1. Recent infection with human A/H1N1 but not A/H3N2 viruses induced significant increases in bovine A/H5N1-neutralizing antibody, as well as increases in NA-binding and NA-inhibiting antibodies to bovine A/H5N1 NA. While the degree of protection afforded by these A/H5N1 cross-reactive antibodies is not known, incorporating NA or enhancing current seasonal vaccine formulations to increase NA-specific antibody titers may increase antibody breadth and protection against both seasonal and pandemic influenza viruses.IMPORTANCEA/H5N1 influenza A viruses continue to pose a pandemic threat to humans. Recent infection of dairy cattle and poultry with A/H5N1 in the USA has magnified that concern. We determined the level of antibodies that recognize A/H5N1 hemagglutinin (HA) and neuraminidase (NA) proteins in a population in Baltimore, MD. We show that while low levels of H5 HA-binding and A/H5N1-neutralizing antibodies are present, there is a significantly stronger recognition of bovine N1 NA. Vaccines that target the N1 NA protein may induce protective antibody responses in humans due to the presence of cross-reactive human N1 NA antibodies.