This study is researching experimental drugs called REGN7508 and REGN9933. The study is focused on participants who have atrial fibrillation, which means that the heart beats too fast and unevenly. REGN7508 and REGN9933 are designed to help stop blood clots forming in patients with atrial fibrillation.

The aim of the study is to see how well REGN7508 and REGN9933 work in patients that get medicine for their atrial fibrillation. The bleeding effects of REGN7508 and REGN9933 will be compared to another medicine (apixaban), which is available on the market to treat and prevent formation of blood clots.

The study is looking at several other research questions, including:

• What side effects may happen from taking REGN7508 or REGN9933
• How well do the study drugs reduce the risk of having a stroke
• How much of REGN7508 or REGN9933 is in the blood at different times
• Whether the body makes antibodies against REGN7508 or REGN9933 (which could make the drugs less effective or could lead to side effects)

Key Inclusion Criteria:

1. Has AF or flutter (paroxysmal or persistent), not felt 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 Thromboembolism (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 Normalization Ratio (INR) <2.5 at the time of randomization if taking warfarin or another Vitamin K Antagonist (VKA)

Key Exclusion Criteria:

1. Has a mechanical heart valve prosthesis (Note: transcatheter aortic valve replacement is not an exclusion)
2. Has known moderate-to-severe mitral stenosis
3. Has had successful ablation therapy or Left Atrial Appendage (LAA) occlusion/exclusion, or planned ablation or LAA occlusion/exclusion as described in the protocol
4. Had an ischemic stroke within 2 days prior to randomization
5. Has estimated Glomerular Filtration Rate (eGFR) <15 mL/min/1.73m^2 within 30 days prior to randomization or on dialysis or expected to be started as described in the protocol
6. Has a history of central nervous system bleeding within 30 days prior to randomization

Note: Other protocol-defined Inclusion/ Exclusion criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf