Impact of Casirivimab-Imdevimab on Severe Acute Respiratory Syndrome Coronavirus 2 Delta Variant Nasopharyngeal Virus Load and Spike Quasispecies

BACKGROUND: The increasing use of monoclonal antibodies (mAbs) to treat coronavirus disease 2019 raises questions about their impact on the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mAb-resistant variants. We assessed the impact of Casirivimab-Imdevimab on SARS-CoV-2 mutations associated with reduced mAb activity in treated patients. METHODS: We measured the nasopharyngeal (NP) viral load and sequenced the haplotypes of spike gene of 50 patients infected with the SARS-CoV-2 delta variant and treated with Casirivimab-Imdevimab using single-molecule real-time sequencing. RESULTS: The NP SARS-CoV-2 viral load of patients treated with Casirivimab-Imdevimab decreased from 8.13 (interquartile range [IQR], 7.06–8.59) log(10) copies/mL pretreatment to 3.67 (IQR, 3.07–5.15) log(10) copies/mL 7 days later (P < .001). Of the 36 patients for whom follow-up timepoints Spike sequencing were available, none of the Spike mutations that reduced mAb activity were detected. CONCLUSIONS: Casirivimab-Imdevimab is an effective treatment for patients infected with the SARS-CoV-2 delta variant. Despite selective pressure on SARS-CoV-2 Spike quasispecies, we detected no key mutations that reduced mAb activity in our patients.