A new study published in The Lancet Infectious Diseases has investigated the genotypic and phenotypic characteristics of respiratory syncytial virus (RSV) breakthrough infections following the widespread use of the monoclonal antibody nirsevimab (Beyfortus) in infants under 1 year old. The research, conducted in France during the 2023-2024 RSV season—the first season nirsevimab was approved for infant use in the European Union and the United States—analyzed viral genome sequences to assess the potential for viral escape.
In France, approximately 210,000 single doses of nirsevimab were administered last year. The study compared 260 full-length RSV viral genome sequences from nirsevimab-treated infants with breakthrough infections to 285 untreated RSV-infected infants. Of the breakthrough infections, 236 (91%) were RSV-A, and 24 (9%) were RSV-B.
Minimal Resistance Observed in RSV-A Infections
The study revealed that no known nirsevimab resistance-associated mutations were found in RSV-A infections. Among the 24 RSV-B infections, only two showed mutations conferring resistance to nirsevimab. These findings suggest that breakthrough infections in France were infrequent, supporting the continued use of the antibody in future RSV seasons.
The authors emphasized the necessity of ongoing collaboration between research and public health agencies to monitor the emergence of resistance as nirsevimab use becomes more prevalent. In a related commentary, researchers highlighted the need for subgroup-specific surveillance strategies to better understand the varying prevalences of monoclonal antibody-escape mutants between RSV-A and RSV-B.
