The purpose of this study is to evaluate the efficacy and safety of ibuzatrelvir with and without remdesivir compared with remdesivir alone for the treatment of symptomatic COVID-19 in adult participants who are severely immunocompromised. This study is being done to learn how the study drug, called ibuzatrelvir, works in treating people with COVID-19 who have a severely compromised immune system. Ibuzatrelvir stops the virus that causes COVID-19 from multiplying in cells and spreading around the body. It may help people to get better and stay out of the hospital.