A Study of Secutrelvir in Participants With Coronavirus Disease 2019 (COVID-19) Who Are at High Risk for Progression to Severe Disease

• Secutrelvir (Drug), also known as S-892216: Secutrelvir will be administered orally as a tablet.
• Placebo (Drug): Placebo will be administered orally as a tablet.

Primary outcomes

• Proportion of Participants Experiencing Any Component of the Following Composite Endpoint: Targeted Medically Attended Visits (tMAVs) Due to Worsening COVID-19, All-cause Hospitalizations, or All-cause Deaths [Baseline through Day 28]

Secondary outcomes

• Time to Sustained Resolution of 14 COVID-19 Symptoms [Baseline through Day 28]
• Proportion of Participants with tMAVs Due to Worsening COVID-19 [Baseline through Day 28]
• Proportion of Participants with All-cause Hospitalization or All-cause Death [Baseline through Day 28]
• Proportion of Participants with Hospitalization Due to Worsening COVID-19 [Baseline through Day 28]
• Time to First Resolution of 14 COVID-19 Symptoms [Baseline through Day 28]
• Time to Sustained Alleviation of 14 COVID 19 Symptoms [Baseline through Day 28]
• Time to Sustained Resolution of Each of the 14 COVID-19 Symptoms [Baseline through Day 28]
• Time to First Alleviation of 14 COVID-19 Symptoms [Baseline through Day 28]
• Change from Baseline in Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Viral Ribonucleic Acid (RNA) Level [Baseline through Day 28]
• Number of Participants Returning to Usual Health [Baseline through Day 28]
• Number of Participants Returning to Usual Activity [Baseline through Day 28]
• Number of Participants Reporting No Symptoms [Baseline through Day 28]
• Change from Baseline in EuroQol 5 Dimensions 5 Levels (EQ-5D-5L) Score [Baseline through Day 28]
• Plasma Concentration of Secutrelvir and Secutrelvir Metabolites [Day 4, Day 8]
• Number of Participants with Treatment-emergent Adverse Events [Baseline through Week 24]

Sex: All, ages: 12 Years to any

Key Inclusion Criteria:

* Must be 12 to <18 years of age (where permitted by local regulations) with a body weight ≥40 kilograms, or ≥18 years of age regardless of body weight, at the time of signing the informed consent form.
* Presence of risk factors for progression to severe COVID-19 at the time of screening.
* Must have ≥1 COVID-19 signs/symptoms that are at least mild in severity, and the symptoms must still be present in the 24 hours prior to randomization.
* Confirmed SARS-CoV-2 infection, as determined by any SARS-CoV-2 test (for example, quantitative reverse transcription polymerase chain reaction, antigen test) approved according to local regulations, of any respiratory tract specimen (for example, oropharyngeal, nasopharyngeal or nasal swab, or saliva), collected ≤72 hours prior to randomization.
* Must be randomized within 72 hours of symptom onset, defined as the time when the first COVID-19 symptom occurs.
* Contraceptive use by women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
* A women of childbearing potential must have a negative urine pregnancy test within 24 hours prior to receiving the investigational intervention.
* Unable or unwilling to take other locally available COVID-19 antivirals (for example, nirmatrelvir/ritonavir, molnupiravir, remdesivir, ensitrelvir).

Key Exclusion Criteria:

* Planned >24 hours hospitalization for any medical procedure through Day 28.
* Documented respiratory infection (for example, influenza, respiratory syncytial virus) other than COVID-19 within the 14 days prior to the screening visit.
* Known history of cirrhosis or liver decompensation (including ascites, variceal bleeding, or hepatic encephalopathy).
* Known history of any of the following abnormalities in the following clinical laboratory tests (within 6 months prior to the screening visit):
* Total bilirubin ≥2*upper limit of normal (ULN) (except for Gilbert's syndrome)
* Aspartate aminotransferase or alanine aminotransferase ≥3*ULN
* Suspected or confirmed COVID-19 unrelated to the current episode within 6 months prior to randomization or, for moderately immunocompromised participants, within 3 months prior to 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-19 or post-acute sequelae of COVID-19 diagnosis.
* Current severely immunocompromised conditions.
* Received any other COVID-19-specific therapies within the following timeframes:
* Antiviral agents (for example, remdesivir, nirmatorelvir/ritonavir, molnupiravir, ensitrelvir) within 30 days or 5 half-lives (whichever is longer) prior to randomization
* Anti-SARS-CoV-2 monoclonal antibodies and COVID-19 convalescent plasma within 6 months prior to randomization

Note: Other protocol-define criteria apply

• Medical Director, Study director, Shionogi Inc.