Anti-spike monoclonal antibodies have proven invaluable in preventing severe outcomes from COVID-19, including hospitalization and death. The rise of the SARS-CoV-2 delta variant begs the question of whether monoclonal antibodies maintain similar efficacy now as they had when the alpha and beta variants predominated, when they were first assessed and approved. We used a retrospective cohort to compare rates of severe outcomes in an epoch in which alpha and beta were predominant compared with delta. A total of 5356 patients were infused during the alpha/beta variant-predominant (n=4874) and delta variant-predominant (n=482) era. Overall, odds of severe infection were 3.0% of patients in the alpha/beta-predominant era compared with 4.9% in the delta-predominant cohort. The unadjusted odds ratio (OR) was higher for severe disease in the delta era (OR, 1.67; 95% CI, 0.96 to 2.89), particularly when adjusted for Charlson Comorbidity Index (adjusted OR, 2.04; 95% CI, 1.30 to 3.08). The higher odds of severe infection could be due to a more virulent delta variant, although the possibility of decreased anti-spike monoclonal antibody effectiveness in the clinical setting cannot be excluded. Research into the most effective strategies for using and improving anti-spike monoclonals for the treatment of emerging variants is warranted.