Inclusion Criteria:

• Willing and able to provide written informed consent, (individuals ≥ 18 years of age) or assent (individuals ≥ 12 and < 18 years of age) prior to performing study procedures. Individuals age ≥ 18 years may be enrolled with the consent of a legal representative where permitted according to local law and approved nationally and by the relevant institutional review board (IRB) or independent ethics committee (IEC). For individuals ≥ 12 and < 18 years of age, a parent or legal guardian must be willing and able to provide written informed consent prior to performing study procedures.
• Either:
  • Age ≥ 18 years (at all sites) or aged ≥ 12 and < 18 years of age weighing ≥ 40 kg (where permitted according to local law and approved nationally and by the relevant IRB or IEC) with at least 1 of the following pre-existing risk factors for progression to hospitalization:
  • Chronic lung disease: chronic obstructive pulmonary disease, moderate-to-severe asthma, cystic fibrosis, pulmonary fibrosis;
  • Hypertension: systemic or pulmonary;
  • Cardiovascular or cerebrovascular disease: coronary artery disease, congenital heart disease, heart failure, cardiomyopathy, history of stroke, atrial fibrillation, hyperlipidemia;
  • Diabetes mellitus: Type 1, type 2, or gestational;
  • Obesity (BMI 30);
  • Immunocompromised state; having a solid organ transplant, blood, or bone marrow transplant; immune deficiencies; HIV with a low CD4 cell count or not on HIV treatment; prolonged use of corticosteroids; or use of other immune weakening medicines;
  • Chronic mild or moderate kidney disease;
  • Chronic liver disease;
  • Current cancer;
  • Sickle cell disease; OR
  • Age ≥ 60 years, regardless of the presence of other pre-existing risk factors for progression.
• SARS-CoV-2 infection confirmed by molecular diagnostics (nucleic acid [e.g., PCR] or antigen testing) ≤ 4 days prior to screening.
• Presence of ≥ 1 symptom(s) consistent with COVID-19 for ≤ 7 days prior to randomization (such as fever, cough, fatigue, shortness of breath, sore throat, headache, myalgia/arthralgia) {Stokes 2020, World Health Organization (WHO) 2020b}.
• Not currently receiving, requiring, or expected to require supplemental oxygen.
• Not currently requiring hospitalization (hospitalization defined as ≥ 24 hours of acute care).
• Participants of childbearing potential who engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception.

Exclusion Criteria:

• Participation in any other clinical trial of an experimental treatment and prevention for COVID-19.
• Prior hospitalization for COVID-19 (hospitalization defined as ≥ 24 hours of acute care).
• Treatment with other agents with actual or possible direct antiviral activity against SARS-CoV-2 or administration of any SARS-CoV-2 (or COVID-19) vaccine.
• Requiring oxygen supplementation.
• ALT or AST ≥ 5 × upper limit of normal (ULN) at screening or within 90 days of screening.
  • Note: if per local practice only ALT is routinely measured, exclusion criteria will be evaluated on ALT alone.
• Creatinine clearance < 30 mL/min at screening or within 90 days of screening using the Cockcroft-Gault formula in participants ≥ 18 years of age or estimated glomerular filtration rate (eGFR) < 30 mL/min/1.73m^2 at screening or within 90 days of screening using the Schwartz formula in participants < 18 years of age.
• Currently breastfeeding (nursing).
• Known hypersensitivity to the study drug, the metabolites, or formulation excipient.
• Use or planned use of exclusionary medications.