Background Peak SARS-CoV-2 viral replication occurs in the upper respiratory tract in presymptomatic and early symptomatic phases. Administration of a monoclonal antibody may be most beneficial in the early time period immediately after symptom onset. Here we describe the effect of early therapy on efficacy in patients receiving ADI. Methods High risk patients with mild or moderate COVID-19 were enrolled in the ADI treatment study (STAMP), with primary endpoint of COVID-19 related hospitalization or all-cause death through Day 29 in patients with disease due to confirmed or suspected SARS-CoV-2 variants other than Omicron. Patients were randomized 1:1 to receive ADI or placebo administered by a single intramuscular (IM) injection. For this subgroup analysis, patients that had received therapy within 3 days of symptom onset were evaluated. Results In the overall population, the study met the primary endpoint demonstrating 66% relative risk reduction of COVID-19 hospitalization or all cause death in 336 patients. Among 261 patients receiving therapy within 3 days of symptom onset (n=133 ADI, n=128 placebo), ADI was associated with a statistically significant reduction in the risk of COVID-19-related hospitalization or all-cause death through Day 29 compared with placebo (4 [3%] vs. 15 [11.7%], standardized risk difference -8%, 95% CI: -14.11, -1.86, p=0.0106), demonstrating a 72% standardized relative risk reduction in favor of ADI. When given as early therapy, ADI provided a greater reduction in viral load from baseline to Day 5 compared with placebo as assessed by saliva samples, with an adjusted least-squares mean difference of -0.97 log10 copies/mL (95% CI: -1.540, -0.391; p=0.0011). No study drug related SAEs, including deaths, and no hypersensitivity reactions were reported. Conclusion Early therapy with a single dose of ADI 300 mg IM provided a 72% reduction in the risk of COVID-19 related hospitalization and all-cause death compared to placebo in high-risk ambulatory patients with mild to moderate COVID-19. Therapy within the first 3 days also led to a greater reduction in viral load compared to placebo and favorable outcomes in patients who are at high risk for progression of disease. Disclosures Kathryn Mahoney, PharmD, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Kristin Narayan, PhD, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Natalia Betancourt, MS, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Yong Li, PhD, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Deepali Gupta, B.Sc, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Pamela Hawn, PharmD, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Pete Schmidt, MD, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds Myra Popejoy, PharmD, Adagio Therapeutics: Employee|Adagio Therapeutics: Stocks/Bonds.