On Mar. 23, 2021, Pfizer announced that it was progressing to multiple ascending doses after completing the dosing of single ascending doses in a Phase 1 study in healthy adults to evaluate the safety and tolerability of an investigational, novel oral antiviral therapeutic for SARS-CoV-2. This Phase 1 trial was being conducted in the U.S.

The oral antiviral clinical candidate PF-07321332, a SARS-CoV2-3CL protease inhibitor, had demonstrated potent in vitro anti-viral activity against SARS-CoV-2, as well as activity against other coronaviruses, suggesting potential for use in the treatment of COVID-19 as well as potential use to address future coronavirus threats.