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

The purpose of this clinical trial is to learn about the safety and effects of the study medicine (called Nirmatrelvir/Ritonavir) for the possible treatment of COVID-19.

Patients with COVID-19 who have more difficulty in fighting against infections have a higher chance of severe illness. Such patients may benefit from longer treatment durations compared to the standard treatment regimen.

The study is seeking participants who:

* Have a confirmed COVID-19 infection
* Are Immunocompromised
* Experience onset of signs/symptoms attributable to the current COVID-19 infection within 5 days prior to screening and ≥1 signs/symptoms attributable to COVID-19 present on the day of randomization.

In addition, this study will also evaluate the efficacy and safety of a second treatment course of nirmatrelvir/ritonavir in people who experience that their COVID-19 is flaring up within 14 days of having taken a 5-day treatment course of nirmatrelvir/ritonavir.

For this group, the study is seeking participants who:

* Have a confirmed COVID-19 infection
* Experience a worsening of signs/symptoms after completing an initial 5-day course of nirmatrelvir/ritonavir
* The worsening of COVID-19 symptoms must occur within 14 days after completion of the initial 5-day course of nirmatrelvir/ritonavir
* Are Immunocompromised
* Experience onset of signs/symptoms attributable to the current COVID-19 infection within 48 hours prior to screening and ≥1 signs/symptoms attributable to COVID-19 present on the day of randomization.

All participants will be taking the study medicine for either 5, 10, or 15 days. The study medication will be taken by mouth 2 times a day. Participants will take part in this study for about 24 weeks. The first dose of study medication is taken at the study site and the rest at home. Selected participants will need to visit the study site at least 10 times during the study.


Eligibility Criteria:
Inclusion Criteria (applicable for both the main population and population with rebound):

* Participants aged 12 years or older and weighing ≥40 kg at screening.
* Immunocompromised
* ≥1 signs/symptoms attributable to COVID-19 present on the day of randomization.

Participants for the main population must have:

- Confirmed SARS-CoV-2 infection as determined by RT-PCR or other acceptable test method in any specimen collected within 5 days prior to randomization for the main study population.

Participants form the rebound population must have:

- Confirmed SARS-CoV-2 infection as determined by RT-PCR or rapid antigen testing in any specimen collected within 24h prior to randomization and collected within 14 days after the completion of the initial 5-day treatment course of nirmatrelvir/ritonavir for the population with rebound.

Exclusion Criteria:

* Current need for hospitalization or anticipated need for hospitalization within 24 h after randomization
* Known medical history of active liver disease
* Known HIV infection with a viral load \>400 copies/mL or taking prohibited medications for human immunodeficiency virus (HIV)
* Receiving dialysis or have known age-specific estimated glomerular filtration rate (eGFR) or estimated creatinine clearance (eCrCl) \<30 mL/min/1.73 m2 at screening as measured by a serum creatinine point of care device
* Oxygen saturation of \<92% on room air obtained at rest within 24 hours prior to randomization
* Current use of any prohibited concomitant medication(s)
* Females who are pregnant and \<14 weeks gestation