OBJECTIVES: To evaluate the impact of neutralizing monoclonal antibody (mAb) 
treatment and to determine whether the selective pressure of mAbs could 
facilitate the proliferation of severe acute respiratory syndrome coronavirus 2 
(SARS-CoV-2) variants with spike protein mutations that might attenuate mAb 
effectiveness.
PATIENTS AND METHODS: We evaluated the impact of mAbs on the nasopharyngeal (NP) 
viral load and virus quasispecies of mAb-treated patients using single-molecule 
real-time sequencing. The mAbs used were: Bamlanivimab alone (four patients), 
Bamlanivimab/Etesevimab (23 patients) and Casirivimab/Imdevimab (five patients).
RESULTS: The NP SARS-CoV-2 viral load of mAb-treated patients decreased from 8.2 
log10 copies/mL before administration to 4.3 log10 copies/mL 7 days after 
administration. Five immunocompromised patients given Bamlanivimab/Etesevimab 
were found to have mAb activity-reducing spike mutations. Two patients harboured 
SARS-CoV-2 variants with a Q493R spike mutation 7 days after administration, as 
did a third patient 14 days after administration. The fourth patient harboured a 
variant with a Q493K spike mutation 7 days post-treatment, and the fifth patient 
had a variant with a E484K spike mutation on day 21. The emergence of the spike 
mutation was accompanied by stabilization or rebound of the NP viral load in 
three of five patients.
CONCLUSION: Two-mAb therapy can drive the selection of resistant SARS-CoV-2 
variants in immunocompromised patients. Patients given mAbs should be closely 
monitored and measures to limit virus spread should be reinforced.