Complete definition of how mutations affect antibodies used to prevent RSV

New antibodies targeting the F protein of respiratory syncytial virus (RSV) have substantially reduced infant hospitalizations. However, viral resistance is a concern: one antibody failed clinical trials due to emergence of a resistant strain, and sporadic resistance mutations to the most widely used antibody (nirsevimab) have been identified in breakthrough infections. Here we define how RSV F mutations affect antibody neutralization. We first provide a biophysical model of how the buffering effect of bivalent IgG binding combines with differences in monovalent Fab potency to explain why nirsevimab resistance mutations are more common in subtype B than subtype A RSV strains. We then perform pseudovirus deep mutational scanning to safely measure how nearly all mutations to F affect its cell entry function and neutralization by the IgG and Fab forms of nirsevimab, clesrovimab, and several other key antibodies. We use these measurements to enable real-time surveillance of RSV sequences for antibody resistance, and identify rare strains with sporadic resistance mutations. Overall, our work improves understanding of the mechanisms by which viral mutations impact antibody neutralization, enables monitoring for natural RSV strains resistant to antibodies of public-health importance, and can help guide development of future antibodies with resilience to viral escape.