Placebo for nirmatrelvir

A Study to Learn About the Study Medicines (Nirmatrelvir Plus Ritonavir) in People Aged 12 Years or Older With COVID-19 and a Compromised Immune System

A Study to Learn About the Medicine Called Nirmatrelvir Used in Combination With Ritonavir in People With Weakened Immune Systems or at Increased Risk for Poor Outcomes Who Are Hospitalized Due to Severe COVID-19

A Study to Learn About a Repeat 5-Day Treatment With the Study Medicines (Called Nirmatrelvir/Ritonavir) in People 12 Years Old or Older With Return of COVID-19 Symptoms and SARS-CoV-2 Positivity After Finishing Treatment With Nirmatrelvir/Ritonavir