Oral Nirmatrelvir for High-Risk, Nonhospitalized Adults with Covid-19 BACKGROUND: ) inhibitor with potent pan-human-coronavirus activity in vitro. METHODS: We conducted a phase 2-3 double-blind, randomized, controlled trial in which symptomatic, unvaccinated, nonhospitalized adults at high risk for progression to severe coronavirus disease 2019 (Covid-19) were assigned in a 1:1 ratio to receive either 300 mg of nirmatrelvir plus 100 mg of ritonavir (a pharmacokinetic enhancer) or placebo every ...