Respiratory syncytial virus (RSV), a negative-stranded ribonucleic acid (RNA) virus belonging to the Pneumoviridae family, is considered the most important cause of acute lower respiratory tract infection (LRTI) in infants and young children. In most patients, RSV results in upper respiratory tract infection (URTI) eliciting common cold-like symptoms, which might last up to 2 weeks, and are usually self-limiting. RSV-related LRTI is a major cause of hospital admissions and death in young children worldwide. Rilematovir is an investigational, small molecule, RSV fusion inhibitor. This study aims to evaluate the efficacy and safety of rilematovir in hospitalized infants and children (greater than or equal to [>=] 28 days to less than or equal to [<=] 5 years) and, subsequent to completion of the neonatal substudy, in hospitalized neonates (born at term, less than [<] 28 days of age) with RSV infection. The study will include a Screening Period, a Treatment Period, and a Follow-up Period. The total study duration for each participant will be approximately 36 days (Screening included). The efficacy assessments include evaluation under the RRS and the safety assessments include evaluations of physical examinations, vital signs, electrocardiograms, clinical laboratory tests, and adverse events.
The purpose of the study is to evaluate the efficacy of rilematovir compared to placebo treatment with respect to the clinical outcome on the RSV Recovery Scale (RRS).