Study to Evaluate Safety and Antiviral Activity of Doses of JNJ-53718678 in Children (>=28 Days to <=3 Years) With Respiratory Syncytial Virus Infection

RSV viral load AUC from immediately prior to first dose of study drug through Day 5 was determined. The RSV viral load was measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay in mid-turbinate nasal swab specimens. As planned, combined data for both the cohorts was collected, analyzed and reported for this outcome measure.

Time frame: Baseline through Day 5

Population: Intent-to-Treat-infected (ITT-i) analysis set included all randomized participants who received at least one dose of study drug and who had a centrally confirmed RSV RNA viral load of greater than or equal to (\>=) 1 log10 copies/mL above the lower limit of quantification (LLOQ) of the RSV RT-qPCR assay at baseline. Analyses on the ITT-i set were performed as randomized.

Body temperature was reported over time (either investigator or caregiver measured).

Time frame: Baseline, Days 3, 5, 8, 14 and 21

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Change from baseline in body temperature (either investigator or caregiver measured) was assessed.

Time frame: Baseline to Days 3, 5, 8, 14 and 21

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Change from baseline in clinician PRESORS scores (for concepts: activity level, sleep disturbance, breathing problems, retractions, tachypnea, feeding problem, cough, nasal secretions, wheezing, dehydration) was assessed. Clinician PRESORS is a questionnaire recording presence and severity of signs and symptoms of RSV disease and consisted of 10-items, each item score ranges from 0 to 3. Overall RSV symptoms summary parameter was derived (mean of the item scores) which also ranges from 0 to 3. The higher the score, the worse the symptom.

Time frame: Baseline, up to Days 3, 5, 8, 14 and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Change from baseline in heart rate was assessed.

Time frame: Baseline to Days 3, 5, 8, 14 and 21

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

PRESORS is a questionnaire recording presence and severity of signs and symptoms of RSV disease (fever, cough, sputum, wheezing, difficulty breathing, nasal congestion, and feeding issues). PRESORS overall RSV symptoms summary parameter consisted of 12-items, each item score ranges from 0 to 3. A summary score was derived (mean of the item scores) which also ranges from 0 to 3. The higher the score, the worse the symptom.

Time frame: Baseline up to Days 3, 5, 8, 14 and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Change from baseline in peripheral capillary oxygen saturation levels was derived based on reported values over time.

Time frame: Baseline to Days 3, 5, 8, 14, and 21

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Change from baseline in respiratory rate was derived based on the reported measurements of respiratory rate over time. The respiratory rate over time was reported by the investigator.

Time frame: Baseline to Days 3, 5, 8, 14 and 21

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Change from baseline in RSV viral load over time was measured by qRT-PCR in the nasal swab specimens collected at the clinic visits and at home. As planned, combined data for both the cohorts was collected, analyzed and reported for this outcome measure.

Time frame: Baseline up to Days 3, 5, 8, 14, and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

AUC (0-24) is defined as area under the plasma concentration-time curve from timepoint 0 hours until 24 hours post dose estimated by population PK model.

Time frame: 0 to 24 hours post dose on Days 1 and 7

Population: PK analysis set included all participants from Cohort-1 who received JNJ-53718678 and for whom at least one PK concentration was reported. Here, 'n' (number analyzed) represents number of participants who were evaluable for specified timepoints.

Duration of ICU stay was derived based on the reported admission/discharge date/time for ICU. Duration defined as total number of hours a participant was in ICU from first dose of study drug until study termination. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who were admitted to ICU and received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

For the subset of participants who received invasive ventilation post dose, duration for invasive ventilation could not be derived by individual type as start/end dates and times were not collected in full to allow breakdown of duration derivation by ventilation type and only overall duration of oxygen supplementation (overall ventilation support) could be derived which is reported in the outcome measure Duration of Supplemental Oxygen. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure could only have been reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above LLOQ of RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

For the subset of participants who received non-invasive ventilation post dose, duration for non-invasive ventilation could not be derived by individual type as start/end dates and times were not collected in full to allow breakdown of duration derivation by ventilation type and only overall duration of oxygen supplementation (overall ventilation support) could be derived which is reported in the outcome measure Duration of Supplemental Oxygen. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure could only have been reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above LLOQ of RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure and 'n' (number analyzed) represents number of participants who were evaluable at specified categories.

Duration of supplemental oxygen was assessed. Duration was defined as total number of hours a participant used supplemental oxygen from first dose of study drug until study termination. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 28

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Cmax is the maximum plasma concentration of JNJ-53718678 estimated by population PK model.

Time frame: Days 1 and 7

Population: PK analysis set included all participants from Cohort-1 who received JNJ-53718678 and for whom at least one PK concentration was reported. Here, 'n' (number analyzed) represents number of participants who were evaluable for specified timepoints.

Percentage of participants who needed hydration and/or feeding by IV Administration or nasogastric tube after the first dose of study drug was assessed. This parameter was only for participants who didn't require supplemental feeding/hydration before first dose of study drug. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 28

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Percentage of participants who required admission to the ICU was assessed. This outcome measure was applicable for those participants that were not in ICU before first dose of study drug. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Percentage of participants who required invasive mechanical ventilation support (example: endotracheal-mechanical ventilation) after first dose of study drug was assessed. This parameter was only for participants who did not require invasive mechanical ventilation support before first dose of study drug. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Percentage of participants who required non-invasive mechanical ventilation support (example: continuous positive airway pressure) after first dose of study drug was assessed. This parameter was only for participants who did not require non-invasive mechanical ventilation support before first dose of study drug. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Percentage of participants who required non-invasive non-mechanical ventilation support (example: nasal cannula) after first dose of study drug was assessed. This parameter was only for participants who did not require non-invasive non-mechanical ventilation support before first dose of study drug. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Percentage of participants who required supplemental oxygen after first dose of study drug was reported. This parameter was only for participants that did not require oxygen supplementation before first dose of study drug. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Time to clinical stability was derived based on vital signs(SpO2 \>= 92%, SpO2 \>=95% on room air) assessments and supplementation end dates as collected. Time to clinical stability=time from initiation of study treatment until time at which following criteria were met: Time to return to age-adjusted normal value for otherwise healthy, pre-RSV infection status for participant with risk factor for severe RSV disease,no more oxygen supplementation in otherwise healthy participant, participant with risk factor for severe RSV disease and no more IV administered/nasogastric tube feeding/hydration supplementation in otherwise healthy participant or pre-RSV status of IV/nasogastric tube feeding/hydration in participant with risk factor for severe RSV disease. As per protocol and study design,this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Time to discharge from hospital was derived from the reported discharge date/time and from first dose date/time. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 28

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Time to end of supplemental oxygen up to 72 hours from first hospital discharge was assessed. Time to end of supplemental oxygen was defined as time (hours) from first dose of study drug to last end date/time of any oxygen supplementation received, but within 72 hours following first hospital discharge. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to end of supplemental oxygen including supplemental oxygen within 72 hours after first hospital discharge (up to Day 28)

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Time to return to age-adjusted normal values from first dose of study drug based on the reported vital signs (respiratory rate, heart rate, SpO2 \>=92%, and SpO2 \>=95%) values was assessed. As per the study protocol and study design, this outcome measure was planned to be analyzed for participants who were hospitalized only. Only participants in Cohort 1 were hospitalized, hence this outcome measure is only reported for Cohort 1.

Time frame: Up to Day 28

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Ctrough is the trough plasma concentration of JNJ-53718678 estimated by population PK model.

Time frame: Days 1 and 7

Population: PK analysis set included all participants from Cohort-1 who received JNJ-53718678 and for whom at least one PK concentration was reported. Here, 'n' (number analyzed) represents number of participants who were evaluable for specified timepoints.

Plasma concentration of JNJ-53718678 was measured for Cohort-2. As per planned analysis in the protocol, PK sampling was performed on either Day 3 or Day 5 for participants receiving twice daily dosing, resulting in one combined timepoint of Day 3 or Day 5. Hence, the data collected on either Day 3 or Day 5 was pooled and is reported here collectively.

Time frame: Once daily dosing: Day 3 and Day 8 pre- or post-dose. Twice daily dosing: Day 1 at least 1 hour post-dose, and Days 3 or 5 (combined in one timepoint) at least 4 hours after morning dose but prior to evening dose

Population: PK analysis set included all participants from Cohort-2 who received JNJ-53718678 and for whom at least one PK concentration was reported. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Heart rate was measured by the investigator over time.

Time frame: Baseline, Days 3, 5, 8, 14 and 21

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

LS mean RSV viral load on Days 3, 8, and 14 was reported. LS mean viral load (log10 copies/mL) was estimated per time point. The difference in RSV viral Load AUC (log10 copies\*day/mL) from immediately prior to first dose of study drug (baseline) through Day 3, 8 and 14 was determined from the model estimating the LS Mean Viral Load per time point, and is presented in the statistical analysis. The RSV viral load was measured by qRT-PCR assay in mid-turbinate nasal swab specimens. As planned, combined data for both the cohorts was collected, and analyzed for this outcome measure at Days 3 and 8, however combined Cohort 1 and Cohort 2 data were not analyzed for this outcome measure at Day 14, due to the premature study termination.

Time frame: Baseline through Days 3, 8, and 14

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Number of participants with emerging variations in the viral genome potentially associated with resistance to JNJ-53718678 was reported. Number of participants with F gene sequencing data available and with emerging genetic variations post-baseline as compared to baseline, considering 24 RSV F protein positions of interest (positions 127, 137, 138, 140, 141, 143, 144, 323, 338, 339, 392, 394, 396, 397, 398, 399, 400, 401, 474, 486, 487, 488, 489, and 517) was reported.

Time frame: Up to Day 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Percentage of participants by health status assessment based on PRESORS caregiver (ObsRO) general question over time was assessed. PRESORS is a questionnaire recording presence and severity of signs and symptoms of RSV disease (fever, cough, sputum, wheezing, difficulty breathing, nasal congestion, and feeding issues). Health status was assessed by a question (how is the child's health now) of PRESORS questionnaire and responses were categorized as: 1) excellent, 2) very good, 3) good, 4) fair, 5) poor, and 6) very poor.

Time frame: Baseline, Days 3, 5, 8, 14, and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above LLOQ of RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure and 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Percentage of participants by return to pre-RSV disease health status assessment based on PRESORS caregiver (ObsRO) general question over time was assessed by a question (Has the child's health returned to normal \[how it was before RSV?\]) of PRESORS questionnaire and responses were categorized as: 1) No, and 2) Yes. PRESORS is a questionnaire recording presence and severity of signs and symptoms of RSV disease (fever, cough, sputum, wheezing, difficulty breathing, nasal congestion, and feeding issues). Below results are reported for category 'Yes'.

Time frame: Baseline, Days 3, 5, 8, 14, and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Percentage of participants by status of RSV symptoms based on PRESORS caregiver (ObsRO) general question over time was assessed. PRESORS is a questionnaire recording presence and severity of signs and symptoms of RSV disease (fever, cough, sputum, wheezing, difficulty breathing, nasal congestion, and feeding issues). Status of RSV symptoms was assessed by a question (how would you rate the child's RSV symptoms now?) of PRESORS questionnaire and responses were categorized as: 1) none, 2) very mild, 3) mild, 4) moderate, 5) severe, and 6) very severe.

Time frame: Baseline, Days 3, 5, 8, 14, and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above LLOQ of RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure and 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Percentage of participants who required (re)hospitalization during treatment and follow-up was assessed. Percentage of participants requiring re-hospitalization following the initial hospital discharge was assessed in Cohort 1 participants (hospitalized cohort) whilst percentage of participants requiring hospitalization after first dose of study drug was assessed in Cohort 2 participants (outpatient cohort).

Time frame: Up to Day 28

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Percentage of participants with abnormal ECG (PR interval \[for age 0 to 2 years: \>150 msec is abnormally high, for age group 2 to \<3.5 years: \<100 msec is abnormally low and \>150 msec is abnormally high\]; QRS interval \[for 0 to 2 years: \>79 msec is abnormally high, for age group 2 to \<3.5 years: \<40 msec is abnormally low and \>79 msec is abnormally high\]; QT interval \[for age 0 to 2 years: \>500 msec is abnormally high, for age group 2 to \<18 years: \<320 msec is abnormally low and \>450 msec is abnormally high\]; RR interval \[for age 0 to 3 months: \<333 msec is abnormally low and \>750 msec is abnormally high; for age group 3 to 12 months: \<400 msec is abnormally low and \>860 msec is abnormally high; for age 1 to 2 years: \<430 msec is abnormally low and \>1000 msec is abnormally high; for age group 2 to \<18 years: \<600 msec is abnormally low and \>1200 msec is abnormally high\]) findings were assessed.

Time frame: Up to Day 28

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable for specified categories (per ECG parameters).

Percentage of participants with abnormal laboratory findings (chemistry and hematology) worst toxicity grade was assessed based on Division of Microbiology and Infectious Diseases (DMID) toxicity grading scale. DMID toxicity grades range from 1 to 4. Grade 1 = mild: transient or mild discomfort (\<48 hours); no medical intervention/therapy required. Grade 2 = moderate: mild to moderate limitation in activity - some assistance may be needed; no or minimal medical intervention/therapy required. Grade 3 = severe: marked limitation in activity, some assistance usually required; medical intervention/therapy required, hospitalizations possible. Grade 4 = life-threatening or death: Extreme limitation in activity, significant assistance required; significant medical intervention/therapy required, hospitalization or hospice care probable.

Time frame: Up to Day 28

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable for the specified categories.

Percentage of participants with acceptability and palatability of the JNJ-53718678 formulation was assessed through a questionnaire asking about the child's reaction when given the medicine, completed by parent(s)/caregiver(s) after last dosing that categorized as 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, and 7) vomited (within 2 minutes of swallowing medicine). Below results are based on response to child took medicine easily.

Time frame: Day 8

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Percentage of participants with adverse events was assessed. An AE is any untoward medical occurrence in clinical study participants administered a medicinal (investigational or non-investigational) product. An adverse event does not necessarily have a causal relationship with the intervention.

Time frame: Up to Day 28

Population: The safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned.

Percentage of participants with categorized change from Baseline in ECG parameters (QT/ QTcB/ QTcF interval) was assessed. Abnormal ECG change from baseline in QTc and QTcB Interval is categorized as borderline QTc change: 30 ms (milliseconds) to \<60 ms, and abnormally high QTc change: greater than \[\>\] 60 ms), and QTcF Interval is categorized as borderline QTc change: 30 ms to \<60 ms, and abnormally high QTc change: \>60 ms.

Time frame: Baseline to Day 28

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants who were evaluable per ECG parameter.

Percentage of participants with MRU (any medical care encounters) was reported.

Time frame: Up to Day 28

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized.

Percentage of participants with undetectable RSV viral load (as measured by qRT-PCR) at each timepoint throughout the study was reported. As planned, combined data for both the cohorts were collected, analyzed and reported for this outcome measure.

Time frame: Baseline, Days 3, 5, 8, 14 and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.

Percentage of participants with vital signs (SBP,DBP, pulse rate, respiratory rate, body temperature and SpO2) abnormalities (abnormally low \[ABL\] and abnormally high \[ABH\]) were reported. DBP: ABL: \<35 mmHg:0-3 months (mths), \<40 mmHg:3 mths- \<3.5 years,ABH: \>65 mmHg:0-3 mths, \>85 mmHg:3-12 mths, \>90 mmHg:1-2 years, \>70 mmHg: 2- \<3.5 years; SBP:ABL: \<60 mmHg:0-12 mths,\<75 mmHg:1-2 years, \<80:2- \<3.5 years,ABH: \>110:0-12 mths, \>120 mmHg:1-2 years, \>10 mmHg:2- \<3.5 years; Pulse rate:ABL: \<80 bpm:0-3 mths, \<70 bpm:3 mths-12 mth,\<60 bpm:1-2 years, \<90 bpm:2- \<3.5 years,ABH: \>180 bpm:0-3 mths, \>150 bpm:3 mths-12 mths, \>140 bpm:1-2 years, \>130:2- \<3.5 years; Respiratory rate:ABL: \<25 bpm:0-3 mths, \<20 bpm: 3 mths-12 mths,\<18 bpm:1-2 years, \<20 bpm:2- \<3.5 years, ABH:\>70 bpm:0-3 mths, \>60 bpm:3 mths-12 mths, \>50 bpm:1-2 years, \>35 bpm:2- \<3.5 years; SpO2: ABL: \<92%: 0-\<3.5 years; Temperature (Celsius): ABH:\>37.8:Tympanic, \>38.0:forehead, oral, axillary, \>37.2:rectal.

Time frame: Up to Day 28

Population: The Safety analysis set included all participants who received at least 1 dose of study agent, and were analyzed as treated, regardless of the randomized treatment group assigned. Here, 'n' (number analyzed) represents number of participants evaluable per vital sign parameter. As per change in planned analysis, the upper limit for the last age group was 3.5 years instead of 3 years.

Percentage of participants with worsening or improvement of RSV disease based on general questions of overall health was assessed. Improvement or worsening was assessed by a question 'Would you say the child's RSV symptoms have improved, are about the same or are worse than when the child entered the study' and responses were categorized as: 1) very much improved, 2) much improved, 3) a little improved, 4) about the same, 5) a little worse, 6) much worse, and 7) very much worse.

Time frame: Days 14 and 21

Population: ITT-i analysis set included all randomized participants who received at least one dose of study drug and who had centrally confirmed RSV RNA viral load of \>=1 log10 copies/mL above the LLOQ of the RSV RT-qPCR assay at baseline. Analyses on ITT-i set were performed as randomized. Here, 'n' (number analyzed) represents number of participants who were evaluable at specified timepoints.