Study of Oral Nirmatrelvir (PF-07321332)/Ritonavir Compared With Placebo in Nonhospitalized Adults Who are at Higher Risk of Severe COVID-19 Illness. An Interventional Efficacy and Safety, Phase 2/3, Double-Blind, 2-Arm Study to Investigate Orally Administered PF- 07321332/Ritonavir Compared With Placebo in Nonhospitalized Symptomatic Adult Participants With COVID-19 Who are at Increased Risk of Progressing to Severe Illness.