Background Given the challenges associated with timely delivery of monoclonal antibody (mAb) therapy to outpatients with coronavirus disease 2019 (COVID-19) who are most likely to benefit, it is critical to understand the effectiveness of such therapy outside the context of clinical trials. Methods This was a case–control study of 1257 adult outpatients with COVID-19, ≥65 years of age or with body mass index (BMI) ≥35, who were entered into a lottery for mAb therapy. Results Patients who were called to be offered mAb therapy had a statistically significant 44% reduction in the odds of hospitalization within 30 days of a positive severe acute respiratory syndrome coronavirus 2 test compared with those who were not called (odds ratio [OR], 0.56; 95% CI, 0.36–0.89; P=.01). Patients who actually received bamlanivimab had a statistically significant 68% reduction in the odds of hospitalization compared with those who did not receive bamlanivimab (OR, 0.32; 95% CI, 0.11–0.93; P=.04). Conclusions This study supports the effectiveness of bamlanivimab in reducing COVID-19-related hospitalizations in patients ≥65 or with BMI ≥35.
