Natural H5N1 immunity in dairy cows is durable and cross-protective but non-sterilizing

ABSTRACT Ongoing transmission of influenza A virus (H5N1) in U.S. dairy cattle threatens both animal and human health, underscoring the need to understand the durability of host immunity against reinfection with evolving genotypes. We challenged naïve and convalescent cows, infected one year prior with H5N1 genotype B3.13, with either homologous B3.13 or heterologous D1.1 genotype virus. Homologous rechallenge resulted in complete clinical protection with no infectious viral shedding. Conversely, heterologous rechallenge led to transient clinical disease and limited infectious viral shedding. Convalescent cows experienced significantly milder disease than naive cows, which developed severe illness with high viral shedding and required early euthanasia, regardless of the strain. These findings indicate that naturally acquired immunity offers strong protection against severe illness but may allow silent transmission of divergent strains. Therefore, natural herd immunity alone is unlikely to eliminate the virus; controlling H5N1 in cattle will likely require vaccination strategies that address viral evolution.