Abstract
An unprecedented outbreak of a highly pathogenic avian influenza virus, H5 clade 2.3.4.4b, was reported in United States dairy cattle during the spring of 2024. It has now spread to hundreds of herds across multiple states. In humans, antibodies to the hemagglutinin (HA) protein confer the strongest protection against infection. Human herd immunity limits viral spread but also drives the emergence of antigenic variants that escape dominant antibody responses. We used store-bought milk to profile the collective H5N1 antibody response of dairy cattle herds. We detected HA binding antibodies in specific samples from states with recent/ongoing outbreaks. These antibodies present in milk neutralized replicating virus expressing dairy cattle HA and neuraminidase (NA). Despite originating from independent vendors, dairies/plants, geographic regions, and time, antibodies present in these samples are remarkably similar in activity and HA binding specificity. The dominant antibody response was clade 2.3.4.4b HA specific, followed by cross-reactivity with other H5s. Whether the uniformity of the response is a pathway to achieve herd immunity or an avenue for antigenic variants to rapidly escape remains to be seen.