Study to Evaluate the Safety and Antiviral Activity of Remdesivir (GS-5734™) in Participants With Severe Coronavirus Disease (COVID-19)

Key Inclusion Criteria: * Willing and able to provide written informed consent, or with a legal representative who can provide informed consent, or enrolled under International Conference on Harmonization (ICH) E6(R2) 4.8.15 emergency use provisions as deemed necessary by the investigator (age ≥ 18), or willing and able to provide assent (age ≥ 12 to \< 18, where locally and nationally approved) prior to performing study procedures * Aged ≥ 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 institutional review board (IRB) or independent ethics committee (IEC)) * Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)-2 infection confirmed by polymerase chain reaction (PCR) test ≤ 4 days before randomization * Currently hospitalized * Peripheral capillary oxygen saturation (SpO2) ≤ 94% or requiring supplemental oxygen at screening Key Exclusion Criteria: * Participation in any other clinical trial of an experimental treatment for COVID-19 * Concurrent treatment with other agents with actual or possible direct acting antiviral activity against SARS-CoV-2 is prohibited \< 24 hours prior to study drug dosing * Evidence of multiorgan failure * Mechanically ventilated \[including veno-venous (V-V) extracorporeal membrane oxygenation (ECMO)\] ≥ 5 days, or any duration of veno-arterial (V-A) ECMO. * Alanine Aminotransferase (ALT) or aspartate aminotransferase (AST) \> 5 X upper limit of normal (ULN) * Creatinine clearance \< 50 mL/min using the Cockcroft-Gault formula for participants ≥ 18 years of age {Cockcroft 1976} and Schwartz Formula for participants \< 18 years of age Note: Other protocol defined Inclusion/Exclusion criteria may apply.