Phase 3 Study of S-217622 in Prevention of Symptomatic SARS-CoV-2 Infection

The purpose of this study is to measure the proportion of participants who are infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (positive reverse transcription polymerase chain reaction \[RT-PCR\] test) and have coronavirus disease 2019 (COVID-19) symptom(s) with S-217622 tablets compared with placebo tablets in participants who are household contacts of an individual with symptomatic COVID-19.

This study will enroll 2 types of participants: 1. those with a negative screening SARS-CoV-2 infection (referred to as "study participants"); 2. those with a positive screening SARS-CoV-2-infection (referred to as "index participants"). Study participants will receive study intervention (S-217622 or placebo), index participants will receive no study intervention.

Eligibility Criteria:
Inclusion Criteria:

Index Participants

Only 1 index participant from each household will be enrolled in the study. Participants are eligible to be included as an index participant if all of the following criteria apply:

1. Pediatric participants or adult participants (of any age)
2. The index participant must:

   1. Have at least 1 COVID-19 symptom within 24 hours before the index participant providing informed consent. COVID-19 symptom(s) must be deemed by the investigator as related to the current SARS-CoV-2 infection (not related to preexisting comorbidities) or deemed as preexisting and worsened due to SARS-CoV-2 infection.
   2. Must have positive SARS-CoV-2 test (a nucleic acid amplification test or antigen test) from any respiratory tract specimen from a sample collected prior to randomization of the participant
3. Must have a potential study participant who can participate in the study after onset of COVID-19 symptoms in an index participant

Study Participants

Multiple study participants from the same household are allowed to be enrolled in the study. The study intervention randomization will be performed based at the study participant's level. Participants will be eligible to be included in the study only if all of the following criteria apply:

1. ≥ 12 years of age at the time of signing the informed consent
2. Has a negative screening for SARS-CoV-2 infection, as determined by SARS-CoV-2 test (a nucleic acid amplification test or antigen test) from any respiratory tract specimen (for example, oropharyngeal, nasopharyngeal or nasal swab, or saliva).
3. Has lived in household with index participant and will continue to live in same household and share common areas such as dining rooms and bathrooms until the end of the study
4. Must not be considered by the investigator or subinvestigator to have SARS-CoV-2 infection and:

   1. No measured fever at Screening (defined as body temperature ≥ 38.0°Celsius \[C\] per tympanic or rectal thermometer or ≥ 37.5°C per axillary, oral, or forehead/temporal thermometer)
   2. No COVID-19 symptoms at Screening (if a participant has COVID-19-like symptoms caused by comorbidities, only 1 symptom will be allowed)
5. Capable and willing to complete a participant diary
6. a. Male participants: Participants with partners that are woman of childbearing potential (WOCBP) must agree to use effective contraception. Participants with pregnant partners must agree to use effective contraception.

   b. Female participants: A female participant is eligible to participate if she is not pregnant or breastfeeding, and the following conditions applies:
   * Is not a WOCBP or
   * All of the following apply:

     * Is a WOCBP and using a contraceptive method that is effective as described in the protocol.
     * A WOCBP must have a negative urine pregnancy test within 24 hours before the first dose of study intervention period.
     * If a urine pregnancy test cannot be confirmed as negative (for example, an ambiguous result), a serum pregnancy test must be negative.
   * Additional requirements for pregnancy testing during and after study intervention as described in the protocol.
   * The investigator/subinvestigator will review medical history, menstrual history, and recent sexual activity to decrease the risk of inclusion of a woman with an early undetected pregnancy.
7. Participant must be randomized ≤ 72 hours from onset of COVID-19 symptoms in the index participant.

Exclusion Criteria:

Study Participants: Participants will be excluded from the study if any of the following criteria apply:

1. Tested positive for SARS-CoV-2 in the past 6 months.
2. Have an underlying disease requiring systemic corticosteroids (excluding topical), antipyretics/analgesics, or immunosuppressive agents.
3. Known current renal impairment defined as creatinine clearance \< 30 milliliters/minute by Cockcroft Gault or requiring dialysis.
4. Participants with severe liver dysfunction, such as known history of cirrhosis or liver decompensation (including ascites, variceal bleeding, or hepatic encephalopathy).
5. Received approved, authorized, or investigational convalescent plasma, or other anti- SARS-CoV-2 biologic products within 3 months of the Screening Visit
6. Received approved, authorized, or investigational anti-SARS-CoV-2 monoclonal antibodies (mAbs) in the last 6 months.
7. Received any SARS-CoV-2 vaccine within 6 months prior to the Screening Visit or is expected to receive a SARS-CoV-2 vaccine or other approved, authorized, or investigational postexposure prophylaxis treatments until the end of the study.
8. Participants who have used any of the following drugs within 14 days prior to enrollment:

   1. Strong cytochrome P450 (CYP) 3A inducer
   2. Products containing St. John's wort
9. Exposed to an investigational drug within 30 days or 5 half-lives of the drug prior to the Screening Visit.
10. Positive urine pregnancy test at Screening Visit or are lactating.
11. Known allergy/sensitivity or any hypersensitivity to components of S-217622 or placebo for S-217622.
12. Any condition or circumstance that, in the opinion of the investigator or subinvestigator, would compromise the safety of the participant or the quality of the study data.

Note: Other protocol-defined Inclusion/Exclusion criteria may apply.