Stroke and Systemic Embolism Prevention in Adult Participants With Atrial Fibrillation

This study is researching an experimental drug called REGN7508 (called "study drug") and how it compares against another treatment called apixaban. The study is focused on people who have atrial fibrillation, which means that the heart beats too fast and unevenly. The aim of the study is to see how safe and effective REGN7508 is in preventing stroke and/or systemic embolism in participants with atrial fibrillation and how it compares against participants that will receive apixaban in this study. The study is looking at several other research questions, including: * What side effects may happen from taking the study drug * Whether the body makes antibodies against the study drug (which could make the study drug less effective or could lead to side effects)

Key Inclusion Criteria: 1. Has AF or flutter (paroxysmal or persistent), not considered to be secondary to a reversible cause, and an indication for indefinite anticoagulation treatment as described in the protocol 2. Meets one of the following: 1. CHA2DS2-VA \[C: Congestive heart failure; H: Hypertension; A2: Age ≥75 years (double points); D: Diabetes mellitus; S2: Stroke or TIA or Systemic embolism (double points); V: Vascular disease; A: Age 65-74 years\] score ≥2 and Oral Anticoagulant (OAC) naïve OR 2. CHA2DS2-VA score ≥3 OR 3. CHA2DS2-VA score of 2 AND at least 1 enrichment criteria as described in the protocol 3. Must have an International Normalized Ratio (INR) \