Human monoclonal antibodies isolated after seasonal vaccination broadly neutralize antigenically drifted influenza B viruses.

Antigenic drift in hemagglutinin (HA) enables influenza viruses to escape host immunity. Elucidating molecular features of antigenic drift is essential for updating seasonal vaccines and pandemic preparedness. Here, we found that influenza B viruses (IBVs) isolated after 2019 escaped neutralization by several previously identified broadly neutralizing monoclonal antibodies (bnAbs). Meanwhile, we identified two IBV bnAbs, CAV-CF22 and CAV-CH76, isolated via quadrivalent vaccine. They exhibited broad neutralizing activity against Victoria- and Yamagata-lineage viruses in vitro and protected in vivo against contemporary Victoria and Yamagata strains. Phylogenetic and structural analysis revealed fixation of K136E in post-2019 Victoria HA, disrupting epitopes targeted by most previously characterized head-directed IBV-monoclonal antibodies (mAbs). High-resolution structures reveal that, rather than engaging K136, CAV-CF22 and CAV-CH76 insert HCDR3 into the receptor-binding site (RBS) to sterically mimic sialic acid. The conservation of epitope residues underlies the antibodies' broad neutralizing activity against IBV and informs antibody- and vaccine-design strategies that are resilient to recent IBV drift.