RRS was an ordinal scale to assess a participant's clinical status. The RRS provided 7 mutually exclusive categories ordered from best (1) to worst (7) where 1 =home without signs/symptoms, 2 =home with sign/symptoms, 3 =ward without supplemental oxygen (O2) or feeding/hydration, 4 =ward with supplemental or feeding/hydration, 5 =intensive care unit (ICU) without mechanical ventilation (included both invasive and non-invasive mechanical ventilation), 6 =required mechanical ventilation and 7=worst (death). Higher category indicates worse condition. With or without signs/symptoms was defined as the key RSV signs/symptoms (breathing problems, retractions, tachypnea, cough, wheezing/breathing sounds, and tachycardia) resolved (absent or mild) or not resolved assessed by parent/caregiver.

Intent to treat - infected (ITT-i) analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Due to the early termination of study, last day of RRS treatment (Day 8) was considered instead of the original planned day defined as when 50% participants would have been discharged.

Clinically resolved was defined as participant required no oxygen supplementation, no supplemental feeding/hydration, no need for ICU and had Key RSV signs/symptoms resolved to absent or mild as per ClinRO signs/symptoms. Clinically resolved Key RSV signs/symptoms were assessed based on clinician's observations as resolved if participant had no retractions, tachypnea, tachycardia, breathing problems (nasal flaring, head bobbing, grunting); cough (resolved if little or no coughing or occasional strong cough or sometimes productive) and wheezing (resolved if no wheezing or terminal expiratory wheezing or only with stethoscope).

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis.

Time (in hours) from first dose of study intervention to first resolution of key RSV signs/symptoms was evaluated based on ObsRO assessment after free of supplementation (O2/feeding/hydration) for at least 24 hours. Clinically resolved was defined as participant required no oxygen supplementation, no supplemental feeding/hydration, no need for ICU and had key RSV signs/symptoms resolved to absent or mild as per ObsRO signs/symptoms. Resolution of key signs/symptoms assessment was based on observations of child's parent/caregiver as resolved if no retractions, tachypnea, tachycardia, breathing problems (gasping for air nostrils, flaring when breathing, head bobbed back and forth when breathing), no breathing sound; cough (no coughing, little coughing without problems). Kaplan-Meier method was used for estimation.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had RSV infection confirmed by central laboratory analysis.

RSV related complications included respiratory complications (respiratory failure, apnoeic attacks, bronchiolitis, bronchial obstruction, pneumonia and asthmatic crisis), infectious complications (otitis media, bacterial respiratory tract infections and sepsis), cardiovascular complications (arrhythmia, cardiogenic shock, hemodynamic instability, congestive cardiac failure), acid-base or electrolyte complications (metabolic acidosis, metabolic alkalosis, hyponatremia, hypokalemia, hyperkalemia, hypocalcemia, hypercalcemia, hypoglycemia and hyperglycemia). Participants were counted only once, regardless of the number of complications they actually experienced.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis.

An adverse event (AE) is any untoward medical occurrence in a clinical study participant administered a medicinal (investigational or non-investigational) product and did not necessarily have a causal relationship with the treatment. A TEAE was defined as an AE with an onset after the initiation study drug (Day 1) up to end of study (Day 35). AEs included both serious and non-serious AEs.

Safety analysis set included all participants who received at least 1 dose of study intervention.

Number of participants with abnormally low (AL) and abnormally high (AH) values of bicarbonate, direct bilirubin, urea nitrogen, basophils, eosinophils, erythrocyte (Ery). mean corpuscular hemoglobin (HGB) concentration (conc), Ery. mean corpuscular hemoglobin, erythrocytes, leukocytes, lymphocytes, monocytes, neutrophils and reticulocytes were reported based on the investigator's discretion.

Safety analysis set included all participants who received at least 1 dose of study intervention. Here, N (Overall number of participants analyzed) signifies participants evaluable for this outcome measure and 'n' (number analyzed) signifies number of participants with available data at each specified category.

Number of participants with abnormally low and abnormally high values of ECG parameters (PR interval and RR interval) as assessed based on the investigator's discretion were reported.

Safety analysis set included all participants who received at least 1 dose of study intervention.

Number of participants with abnormally low and abnormally high values of vital signs from baseline were assessed based on investigator's discretion. Vital signs included systolic blood pressure (SBP) (millimeter of mercury [mmHg]), diastolic blood pressure (DBP) (mmHg), pulse rate (beats per minute), respiratory rate (breaths per minute), temperature (degree Celsius) and oxygen saturation (in percentage).

Safety analysis set included all participants who received at least 1 dose of study intervention. Here, 'n' (number analyzed) signifies number of participants with available data at each specified category.

Percentage of participants requiring ICU stay was analyzed and reported.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants with no ICU stay before first dose of rilematovir.

Duration (in hours) of ICU stay was defined as total number of hours a participant experienced an ICU stay from first dose of rilematovir until study termination, calculated as the sum of all separate records of ICU stay.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants who required ICU stay before first dose of rilematovir and continued after first dose plus participants who required ICU stay after first dose of drug without prior ICU stay.

Percentage of participants requiring re-hospitalization (participants re-hospitalized [ward or ICU] after been discharged from hospital) for respiratory/other reasons were reported.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis.

Percentage of participants requiring any type of oxygen supplementation (invasive mechanical ventilation, non-invasive mechanical ventilation and non-invasive non-mechanical ventilation) were reported.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (overall number of participants analyzed) signifies participants with no use of oxygen supplementation before first dose of rilematovir.

Duration (in hours) of oxygen supplementation was defined as total number of hours a participant used supplemental oxygen from either prior to first dose and/or after first dose of drug until study termination, calculated as the sum of all separate records of supplementation.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants who required oxygen supplementation before first dose of rilematovir and continued after first dose plus participants who required oxygen supplementation after first dose of rilematovir without prior oxygen supplementation.

Percentage of participants requiring any type of hydration and/or feeding by intravenous (IV) administration or nasogastric tube or percutaneous endoscopic gastrostomy was reported.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants with no use of feeding/hydration supplementation before 1st dose of study drug.

Duration (in hours) of supplemental feeding/hydration was defined as total number of hours a participant was administered feeding/hydration supplementation from either prior to first dose and/or after first dose of drug until study termination, calculated as the sum all separate records of supplementation use per participant.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants who required supplemental feeding/hydration before first dose of rilematovir and continued after first dose plus participants who required supplemental feeding/hydration after first dose of rilematovir without prior supplemental feeding/hydration.

Medical resource utilization was assessed by medical care encounters and treatments. Medical encounters and treatments included physician or emergency room visits, tests and procedures, and medications, surgeries and other selected procedures, inpatient and outpatient.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis.

Antiviral activity was determined based on measurements of RSV viral load which was measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in the mid-turbinate (MT) nasal swab specimens.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, 'n' (number analyzed) signifies number of participants with data evaluable at each specified time point.

Antiviral activity was determined based on measurements of RSV viral load which was measured by qRT-PCR in the MT nasal swab specimens.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants evaluable for this outcome measure and 'n' (number analyzed) signifies number of participants with data evaluable at each specified time point.

Percentage of participants with undetectable RSV viral load was analyzed.

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, 'n' (number analyzed) signifies number of participants with data evaluable at each specified time point.

Plasma concentrations of rilematovir were assessed. Participant wise data were reported for this outcome measure.

Pharmacokinetics analysis set (PKAS) included participants who had received at least 1 dose of rilematovir and had at least 1 valid blood sample drawn for pharmacokinetics analysis. No summary analysis was done as study was terminated early and participant wise data were reported. Here, "n" (Number analyzed) signifies specific participant with data evaluable at each specified timepoint.

Acceptability and palatability were assessed by clinician electronic clinical outcome assessment (eCOA) questionnaire which consisted of 7 questions, 1- child took medicine easily, 2- disgusted expressions after tasting medicine, 3- cried after tasting medicine, 4- would not open mouth or turned head away to avoid medicine, 5- spit out or coughed out medicine, 6- gagged, 7- vomited (within 2 minutes of swallowing medicine).

ITT-i analysis set included all randomized participants who received at least 1 dose of study intervention and had an RSV infection confirmed by central laboratory analysis. Here, N (Overall number of participants analyzed) signifies participants evaluable for this outcome measure.