The purpose of this clinical trial is to learn about the safety, pharmacokinetics (pharmacokinetics helps us understand how the drug is changed and eliminated from your body after you take it), and efficacy (how well a study treatment works in the study) of the study medicine (called nirmatrelvir/ritonavir) for potential treatment of coronavirus disease 2019 (COVID-19).

The study medicine will be given to patients under 18 years of age with COVID-19 that are not hospitalized but are at risk for severe disease.

Inclusion criteria:

• Male and female, age 0 to < 18 years, able to swallow for some participants
• Confirmed SARS-CoV-2 infection within 72 hours prior to enrollment
• Initial onset of COVID-19 signs/symptoms within 5 days prior to the day of enrollment and at least 1 of the specified COVID-19 signs/symptoms present at enrollment
• Has at least 1 characteristic or underlying medical condition associated with an increased risk of developing severe illness from COVID-19

Exclusion Criteria:

• History of or need for hospitalization for the medical treatment of COVID-19
• Total bilirubin >=2X upper limit of normal (ULN) (except for Gilbert's syndrome)
• Receiving dialysis or have known moderate to severe renal impairment
• Suspected or confirmed concurrent active systemic infection other than COVID-19
• History of hypersensitivity or other contraindication to any of the components of the study intervention
• Current or expected use of any medications or substances that are highly dependent on CYP3A4 for clearance or strong inducers of cytochrome P450 (CYP)3A4
• Has received or is expected to receive antibody treatment, antiviral treatment or convalescent COVID-19 plasma
• Participating in another interventional clinical study with an investigational compound or device, including those for COVID-19 through the study follow up
• Females who are pregnant or breastfeeding

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.