Respiratory Syncytial Virus Strain Evolution and Mutations in Western Australia in the Context of Nirsevimab Prophylaxis.

Background: Nirsevimab is a long-acting monoclonal antibody used to prevent respiratory syncytial virus (RSV) infection in infants and high-risk children. During the 2024 RSV season in Western Australia, 21 922 doses were administered to infants entering their first season and 1221 doses to at-risk children. In this context, the selection and spread of escape variants are a potential concern. This study aimed to investigate nirsevimab binding site mutations using clinical and wastewater data. Methods: We performed whole genome sequencing on 382 clinical RSV samples and 12 wastewater samples collected between September 2023 and October 2024. RSV subtypes, genetic diversity, and mutations within the nirsevimab binding region of the F protein were analyzed. Phylogenetic analysis was conducted to assess lineage dynamics and the potential emergence of escape variants. Results: RSV-A was the dominant subtype (61.8%), with RSV-B accounting for 38.2% of cases. No lineage shifts were observed following nirsevimab introduction, and none of the known mutations associated with high-level nirsevimab resistance were detected in either clinical or wastewater samples. The prevalent RSV-B mutation combination (F:I206M:Q209R:S211N) was observed consistently but is not associated with reduced nirsevimab efficacy. Wastewater sampling, covering approximately 2 million people from the Perth metropolitan region, confirmed findings from clinical sequences, reinforcing the absence of resistance mutations. Conclusions: No evidence of nirsevimab escape mutations was found in clinical or wastewater samples during the 2024 RSV season. Continued genomic surveillance, including wastewater monitoring, is essential to detect emerging resistance and ensure the long-term efficacy of prophylactic interventions.