Mutational constraints on RSV F and its neutralization by antibodies.

New antibodies targeting the F protein of respiratory syncytial virus (RSV) have substantially reduced infant hospitalizations1. However, viral resistance is a concern: one antibody failed clinical trials because of a resistant strain2, and sporadic resistance mutations to the most widely used antibody (nirsevimab) have been identified3-6. Here we define how RSV F mutations affect antibody neutralization. We first provide a biophysical model of how the buffering of bivalent IgG binding combines with the lower Fab potency of nirsevimab to subtype B to make resistance to this antibody more common in subtype B than A 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 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 show that resistant strains have arisen sporadically but are at present rare. Overall, our work shows how Fab potency and epitope specificity combine to determine how viral mutations affect 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.