Study to evaluate the safety and antiviral activity of remdesivir (GS‐5734™) in participants with severe coronavirus disease (COVID‐19)  Drug: remdesivir, standard of care  United States, Hong Kong/COVID‐19  400, all, 18 years and older  Treatment  Experimental: demdesivir (RDV), 5 days participants will receive continued standard of care therapy together with RDV 200 mg on Day 1 followed by RDV 100 mg on Days 2, 3, 4, and 5