RSV Monoclonal Antibody Resistance: What Clinicians Need to Know

This episode addresses the clinically important topic of resistance to monoclonal antibody prophylaxis. Dr. Simões provides a detailed historical context, describing how an earlier pre-fusion-specific monoclonal antibody that never reached market was ultimately shown to be ineffective against a resistant RSV B strain that emerged in China and spread globally during clinical trials — without any immune pressure from the agent itself. This historical example illustrates how resistance can arise and spread independent of drug use.

Turning to currently approved agents, Dr. Simões describes a concerning finding from surveillance in France, where after approximately 200,000 doses of one monoclonal were administered, 9% of RSV B strains isolated from treated patients carried resistance mutations. These mutations were not found in untreated controls. The theoretical vulnerability of pre-fusion-only agents is contrasted with dual pre/post-fusion binding (which has not generated resistance over more than two decades of use with palivizumab). Dr. Simões clarifies that resistance mutations were detected in trials of both currently approved agents, and while the risk remains theoretical in widespread practice, active genomic surveillance is underway globally.

The practical takeaway for clinicians is one of vigilant awareness rather than alarm: the risk of resistance developing to maternal immunization is essentially zero, and existing prophylaxis strategies provide benefits that far outweigh theoretical resistance concerns. The FDA and global regulatory agencies are actively monitoring the situation.

In the next episode, "RSV Efficacy vs. Effectiveness: What Clinical Trial and Real-World Data Reveal," Dr. Creech reviews the clinical trial results and post-licensure effectiveness data for both preventive modalities, helping clinicians understand what these interventions can realistically achieve.