The main purpose of this study is to investigate the antiviral effect of S-337395 compared with placebo among nonhospitalized adult participants with high-risk factors for progression to severe RSV infection starting intervention within 72 hours of RSV symptom onset.

Key Inclusion Criteria:

• Participants who have new onset or worsening (if present chronically) of at least 1 of the following signs and/or symptoms consistent with a viral acute respiratory infection within 72 hours prior to randomization: fever, nasal congestion, nasal discharge, sore throat, cough, sputum production, shortness of breath, or wheezing.

• Participants diagnosed with RSV infection preferably using a rapid polymerase chain reaction (PCR) or other molecular-based diagnostic assay.

• Has at least 1 of the following risk factors for severe RSV disease:

  1. ≥ 75 years of age;
  2. Chronic lung disease that is symptomatic and requiring chronic treatment; and
  3. Chronic cardiovascular disease that is symptomatic and requiring chronic treatment.

• With the exception of the RSV disease, medically stable on the basis of medical history, physical examination, vital signs, and 12-lead electrocardiogram (ECG) performed at screening.

Key Exclusion Criteria:

• Hospitalized or expected to be hospitalized within 24 hours of screening.
• Is considered by the investigator to be immunocompromised, due to an underlying medical condition or medical therapy, chemotherapy, radiation, stem cell or solid organ transplant.
• Has known allergies, hypersensitivity, or intolerance to S-337395 or to any of the excipients of the S-337395 or placebo formulation.
• Suspicion or known severe renal impairment.
• Any other medical or psychiatric condition making the participant unsuitable for the current study or interfering with the evaluation of response to the study intervention in the opinion of the investigator/subinvestigator.
• Has received a therapy intended to treat RSV infection within 14 days prior to screening.
• Is receiving chemotherapy or immunotherapy for malignancy.
• Has received RSV vaccination within 7 days prior to screening.
• Has had either confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or influenza infection (test was positive) during the 28 days prior to screening.
• Has any other confirmed clinically relevant respiratory infection by any pathogen at, or within 28 days of screening.

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