Abstract
Highly pathogenic avian influenza H5Nx viruses are an emerging threat for global health, especially clade 2.3.4.4b H5N1 virus which causes panzootic infections. Here we describe the isolation and characterization of broadly cross-neutralizing monoclonal antibodies (mAbs) against diverse H5Nx viruses from individuals who received a monovalent H5N1 vaccine 15 years ago. By screening over 500 mAbs, we identified 5 mAbs that neutralized the majority of H5 clades including 2.3.4.4b and target three distinct conserved epitopes within the HA globular head. Cryo-electron microscopy structures of these mAbs in complex with HA, deep mutational scanning and neutralization escape studies define the sites of vulnerability of H5 HA. These mAbs mediated stronger prophylactic protection against clade 2.3.4.4b H5N1 infection in mice than the best-in-class mAb targeting the HA stem. Our study identified several highly potent broadly neutralizing H5 mAbs from humans that either alone or in combination provide a pragmatic pandemic preparedness option against the threat of panzootic H5N1 influenza.