A Study to Investigate the Efficacy and Safety of Zelicapavir Compared With Placebo in Adults With RSV Who Are at High Risk for Severe Disease

Inclusion Criteria: * Age ≥75 years at the time of enrollment, or, age ≥18 at the time of enrollment and has at least one of the following conditions that predispose them to complications of RSV infection: 1. CHF 2. COPD * New onset or worsening of preexisting symptoms consistent with respiratory tract infection no more than 72 hours before administration of the first dose of study intervention: feeling feverish, headache, neck pain, fatigue, loss of appetite, interrupted sleep, body aches, sore throat, nasal congestion, cough, cough with phlegm, wheezing, or short of breath. * Reports at least 2 of the following symptoms, one of which must be reported by the participant as at least moderate in severity using the RiiQ™: cough, cough with phlegm, wheezing, or short of breath. * Tested positive for RSV infection defined as a positive nucleic acid amplification test (PCR or other) or rapid-antigen viral detection test performed on a nasal swab specimen collected no more than 72 hours before administration of the first dose of study intervention. * A woman of childbearing potential who is sexually active with a male must agree to use two effective methods of contraception from the date of Screening until 30 days after her last dose of study drug. * A male subject who has not had a vasectomy and is sexually active with a woman of childbearing potential must agree to use effective contraception from the date of Screening to 90 days after his last dose of study drug. Exclusion Criteria: * Reported history of respiratory infection (other than RSV) within 7 days prior to signing the ICF. * Anticipated need for hospitalization within 24 hours of signing the ICF. * Receiving dialysis or have known severe renal impairment. * Reported history of chronic liver disease, including cirrhosis, autoimmune hepatitis, end stage liver disease, history of hepatocellular carcinoma, or decompensated liver failure. * Prior receipt of or waiting to receive a bone marrow, stem cell, or solid organ transplant. Currently taking, or has taken within 7 days prior to randomization, or is anticipated to receive during the study, systemic medications that are known to cause immunosuppression. Note: Chronic use of immunomodulators such as dupilumab and omalizumab is permitted.