The primary objective of the study is to determine whether aspirin alone or aspirin combined with various doses of rivaroxaban causes subclinical GI blood loss as determined by the HemoQuant assay.

The secondary objective of the study is to evaluate the safety and tolerability of aspirin alone or in combination with rivaroxaban to healthy volunteers.

Key Inclusion Criteria:

1. Has a body mass index between 18 and 32 kilograms per metered square (kg/m2), inclusive
2. Is judged by the investigator to be in good health based on medical history, physical examination, vital sign measurements, and electrocardiograms (ECGs) performed during screening
3. Is in good health based on laboratory safety testing obtained at the screening visit
4. Willing and able to abstain from alcohol use for the duration of the study

Key Exclusion Criteria:

1. History of anemia, abnormal bleeding (including vaginal bleeding or excessive menstrual periods), previous diagnosis of bleeding diathesis, or blood clots as defined in the protocol

2. History, in the last year, of any bleeding GI lesions such as peptic ulcer, hemorrhoids or anal fissure, or a positive fecal occult blood test performed either during screening or on Day 1

3. Presence of irregular stool patterns, constipation, or frequent diarrhea that, in the opinion of the investigator, may interfere with stool collections required by the protocol

4. Inability to refrain during study period from activities with high risk of bleeding or trauma such as planned surgery, contact sports, etc.

5. Hemoglobin levels below the lower limit of normal as defined by local laboratory at screening; the lab may be repeated once if initially abnormal

6. PT and aPTT values above the upper limit of normal as defined by the local laboratory during screening; the lab may be repeated once if initially abnormal

7. Platelet count below the lower limit of normal as defined by the local laboratory during screening; the lab may be repeated once if initially abnormal

8. History of clinically significant respiratory, hepatic, renal, GI, endocrine, hematological, psychiatric or neurological disease, as assessed by the investigator that may confound the results of the study or poses an additional risk to the participant by study participation.

   History of any atherosclerotic cardiovascular disease

9. Presents any concern to the study investigator that might confound the results of the study or poses an additional risk to the participant by their participation in the study

10. Has received a COVID-19 vaccination within 1 week of day 1 of the study or for which the planned COVID-19 vaccinations would not be completed 1 week prior to day 1 of the study.

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, has made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry), has the legal authority to share the data, and has 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