Pre-existing neutralizing antibodies against cattle-transmitted influenza A virus H5N1 are detectable in unexposed individuals

The transmission of influenza A virus H5N1 clade 2.3.4.4b from cattle to humans highlights the risk of an H5N1 pandemic. Pre-existing immunity strongly impacts the course and severity of viral infections, making detailed knowledge of antibodies against the spilled-over strain crucial. Here, we assessed humoral immunity against H5N1 A/Texas/37/2024 in H5N1-naive individuals. We performed complementary binding and neutralization assays on 66 individuals and ranked activities among a panel of 76 influenza A virus isolates. We detected low but distinct cross-neutralizing titers against A/Texas/37/2024, with a 3.9- to 15.6-fold reduction compared with selected H1N1 or H3N2 strains. By cloning and characterizing 136 memory B cell-derived monoclonal antibodies, we identified potent A/Texas/37/2024-neutralizing antibodies in five out of six individuals we investigated. These antibodies cross-neutralized H1, competed with antibodies targeting the hemagglutinin (HA) stem, and protected mice from lethal H5N1 challenge. Our findings demonstrate partial pre-existing humoral immunity to A/Texas/37/2024 in H5N1-naive individuals.