Background: Subcutaneous (SC) administration of the monoclonal antibody combination casirivimab and imdevimab (CAS+IMD) decreased the risk of COVID-in a phase 3 trial of uninfected adolescents/adults who shared a household with a SARS-CoV-2–infected individual (NCT04452318) . Here, we present a subgroup analysis of this study, assessing the efficacy and safety of CAS+IMD in preventing symptomatic COVID-in individuals with diabetes and other risk factors for progression to severe disease. Methods: Healthy individuals aged ≥12 years, identified within 96 hours of collection of a positive test from a household contact, were randomized 1:1 to receive SC CAS+IMD 1200 mg or placebo. The primary endpoint was the proportion of participants who developed symptomatic COVID-during the 28-day efficacy assessment period among those who were SARS-CoV-2 RT-qPCR negative at baseline. Post hoc analyses assessed efficacy and safety by diabetes history, body mass index (BMI) , and the presence/absence of ≥1 high-risk factor for severe COVID-19. Results: The efficacy analysis included 2067 participants who were enrolled from July 13, 2020–January 28, 2021 and were RT-qPCR negative at baseline. In participants with diabetes (n=139) , the relative risk reduction (RRR) of developing symptomatic COVID-with CAS+IMD vs. placebo was 76.3%. Similar results were observed in participants with BMI ≥30 kg/m2 (n=728; RRR=74.9%) and in those with ≥1 high-risk factor for severe COVID-at baseline (n=620; RRR=77.5%) . Across subgroups, injection-site reaction was more common with CAS+IMD versus placebo whereas asymptomatic COVID-19, COVID-19, and headache were more common with placebo versus CAS+IMD. Conclusions: In study participants with diabetes and those with other risk factors for severe COVID-19, treatment with SC CAS+IMD decreased the risk of developing symptomatic COVID-19, consistent with results in the overall population. Disclosure M.O'brien: Employee; Regeneron Pharmaceuticals Inc., Other Relationship; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. R.V.Barnabas: Other Relationship; Regeneron Pharmaceuticals Inc. M.S.Cohen: Other Relationship; COVPN, Fogarty, HPTN, McGill, National Institutes of Health, Prime Global Options, Regeneron Pharmaceuticals Inc. A.Mahmood: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. G.A.Herman: Employee; Regeneron Pharmaceuticals Inc., Other Relationship; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. G.D.Yancopoulos: Employee; Regenacy Pharmaceuticals, Inc., Other Relationship; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. D.M.Weinreich: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. The covid-phase 3 prevention trial team: n/a. E.Forleo-neto: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. E.Oviedo orta: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. J.Mcginniss: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. P.Hou: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. F.Isa: Employee; Regeneron Pharmaceuticals Inc., Other Relationship; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. K.Chan: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. N.Sarkar: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. B.J.Musser: Employee; Regeneron Pharmaceuticals Inc., Stock/Shareholder; Regeneron Pharmaceuticals Inc. Funding Supported by Regeneron Pharmaceuticals, Inc., and F. Hoffmann-La Roche Ltd. This trial was conducted jointly with the COVID-Prevention Network, which was funded by the National Institute of Allergy and Infectious Diseases (NIAID) , National Institutes of Health (NIH) .