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

Inclusion Criteria: * Meeting 1 of the 2 categories of COVID-19 risk: * Category A: Immunocompromised * Category B: Non-Immunocompromised, but with ≥2 risk factors * Onset of signs/symptoms attributable to COVID-19 ≤10 days prior to the day of randomization for non-immunocompromised participants (Category B). * Confirmed SARS-CoV-2 infection as determined by Reverse transcription polymerase chain reaction (RT-PCR) or acceptable test method performed by a health care provider in any specimen collected within 48 hours prior to randomization. * Hospitalized for inpatient care for the treatment of clinical manifestations of severe COVID-19. * Requirement for oxygen supplementation (via nasal cannula, mask, non-invasive ventilation \[NIV\] or high flow oxygen) to maintain SpO2 ≥94% at the time of Screening and Randomization. Exclusion Criteria: * Critical illness, defined by ≥1 of the following: * Requirement for mechanical ventilation or extracorporeal membrane oxygenation (ECMO) at randomization, or likely to require intermittent mandatory ventilation (IMV) or ECMO within 12 hours of randomization . * Multi-organ dysfunction/failure. * Hemodynamically unstable, eg. septic shock, cardiac failure or requiring vasopressors. * Participant not expected to survive 24 hours from time of randomization. * History of severe chronic liver disease * Receiving dialysis of any kind or severe renal impairment * Use of nirmatrelvir/ritonavir as an outpatient to treat the current COVID-19 rel…