Key Inclusion Criteria:

• Must be 12 to < 18 years of age (where permitted by local regulation) with a body weight ≥ 40 kilograms (kg), or ≥ 18 and < 75 years of age regardless of body weight, at the time of signing the informed consent form (ICF).
• Must have ≥ 1 of the protocol-defined COVID-19 symptoms of moderate or severe, present within 24 hours prior to randomization.
• Confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection as determined by any SARS-CoV-2 test, collected ≤ 48 hours prior to randomization.
• Must be randomized within 48 hours of COVID-19 symptom onset, defined as the time when the first COVID-19 symptom occurs.
• Agrees not to participate in another clinical study for the treatment of COVID-19 until completion of the study.

Key Exclusion Criteria:

• Current need or anticipated need for hospitalization within 24 hours, due to signs of severe COVID-19 illness or due to other medical conditions requiring hospitalization in the opinion of the site investigator.
• Documented respiratory infection (for example, influenza, respiratory syncytial virus infection) other than COVID 19 within 14 days prior to the screening visit.
• Known history of cirrhosis or liver decompensation (including ascites, variceal bleeding, or hepatic encephalopathy).
• Suspected or confirmed COVID-19 outside of the current episode within 3 months of randomization.
• Suspected or confirmed concurrent active systemic infection other than COVID-19 that may interfere with the evaluation of response to the investigational intervention.
• Ongoing long COVID or postacute sequelae of COVID (PASC) diagnosis.
• Current severely immunocompromised conditions.
• Received or expected to receive any other COVID-19-specific therapies.
• Have previously received secutrelvir.
• Exposed to an investigational drug within 30 days or 5 half-lives (whichever is longer) prior to the screening visit.

Note: Other protocol-defined inclusion and exclusion criteria may apply.