Background Given the challenges associated with timely delivery of monoclonal antibody (mAb) therapy to outpatients with 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 Case-control study of 1257 adult outpatients with Covid-19, ≥ 65 years of age or with 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 SARS-CoV-2 test compared with those who were not called (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=0.04). There was no statistically significant difference in the odds of death between patients who were called and patients who were not called (OR 1.79, 95% CI [0.52, 6.17], P=0.35), but there was a statistically significant difference in the odds of death between patients who received bamlanivimab and those who did not (OR 0.03, 95% CI [0, 0.25), P<0.01). Conclusions This study supports the effectiveness of bamlanivimab in reducing Covid-19 related hospitalizations in patients ≥ 65 or with BMI ≥ 35 and suggests a mortality benefit.

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