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

Study Overview
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Brief Summary
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The purpose of this study is to evaluate the antiviral activity, clinical outcomes, safety, tolerability, and pharmacokinetic/pharmacodynamic relationships of different oral dose levels of JNJ-53718678 in children greater than or equal to 28 days and less than or equal to 3 years of age with respiratory syncytial virus (RSV) disease (hospitalized participants [Cohort 1] or outpatients [Cohort 2]).

Detailed Description
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JNJ-53718678 is an investigational respiratory syncytial virus (RSV) specific fusion inhibitor and is under development for the treatment of RSV infection, which results in an upper and/or lower respiratory tract illness. The primary hypothesis of this study is that JNJ-53718678 has antiviral activity against RSV (that is, results in a decrease in RSV nasal viral load from immediately prior to first dose of study drug until Day 5). This will be assessed by a positive dose-response relationship of JNJ-53718678 compared to placebo. Besides the RSV nasal viral load through day 5, other timepoints will also be evaluated as well as other nasal viral load related parameters. In addition, the evolution of signs and symptoms of RSV disease will be evaluated. Participants' safety will be monitored throughout the study by evaluating the occurrence and severity of adverse events and by laboratory and electrocardiogram measurements. Study participants will be identified when they are hospitalized or expected to be hospitalized within 24 hours after presentation to the hospital (Cohort 1) or present for medical care as outpatients (Cohort 2) with symptoms of an acute respiratory illness supporting a diagnosis of RSV infection. Eligible participants will be randomized 1:1:1 to receive either a low or a high dose of JNJ 53718678 or placebo and will be receiving study treatment for 7 days. They will be followed up for 3 weeks after the last dose. The total study duration for each participant will be approximately 29 days.

Official Title
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A Phase 2, Double-blind, Placebo-controlled Study to Evaluate the Antiviral Activity, Clinical Outcomes, Safety, Tolerability, and Pharmacokinetic/Pharmacodynamic Relationships of Different Doses of JNJ-53718678 in Children >=28 Days and <=3 Years of Age With Acute Respiratory Tract Infection Due to Respiratory Syncytial Virus Infection

Conditions
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Respiratory Syncytial Virus Infections

Intervention / Treatment
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* Drug: JNJ-53718678
* Drug: Placebo


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Informed consent form (ICF) must be given Laboratory diagnosis of respiratory syncytial virus (RSV) infection The participant has an acute respiratory illness The time of onset of RSV symptoms to the anticipated time of randomization must be less than or equal to (<=) 5 days Except for the RSV-related illness, the Participant must be medically stable in case of allowed co-morbid conditions The participant must have been assessed per local public health practice and considered not to have Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection during this respiratory infection Exclusion Criteria: The participant is less than (<) 3 months postnatal age at screening and was born prematurely (i.e, <37 weeks and 0 days of gestation) OR if the participant weighs <2.4 kilogram (kg) or greater than (>) 16.8 kg Participant is considered by the investigator to be immunocompromised within the past 12 months Participant unwilling or unable to undergo mid-turbinate nasal swab procedures Participant is receiving chronic home oxygen therapy at screening Participant has other clinically significant abnormal electrocardiogram (ECG) findings not consistent with the present risk factor for severe RSV disease (if applicable) in the study population, as judged by the investigator based on the machine read ECG results at screening The participant has a QTcF interval greater than (>)450 millisecond (ms) per the machine read (mean of triplicate) parameter result confirmed by repeat triplicate ECG recording during screening

Ages Eligible for Study
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Minimum Age: 28 Days
Maximum Age: 3 Years

Sexes Eligible for Study
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All

Accepts Healthy Volunteers
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No

Study Plan
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How is the study designed?
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Design Details

Primary Purpose: Treatment
Allocation: Randomized
Intervention Model: Parallel Assignment
Masking: Double


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Experimental: High Dose: JNJ-53718678<br>Participants will be randomized to receive JNJ-53718678 2.5 milligram per kilogram (mg/kg) (Age Group 1: greater than or equal to [>=] 28 days and less than [<] 3 months of age), 3 mg/kg (Age Group 2: >=3 months and <6 months of age) and 4.5 mg/kg (Age Group 3: >=6 months and less than or equal to [<=3] years of age) orally twice daily for 7 days. | Drug: JNJ-53718678<br>* Participants will receive JNJ-53718678 (high dose or low dose) orally twice daily for 7 days.<br>|
| Experimental: Low Dose: JNJ-53718678<br>Participants will be randomized to receive JNJ-53718678 0.85 mg/kg (Age Group 1: >=28 days and <3 months of age), 1 mg/kg (Age Group 2: >=3 months and <6 months of age) and 1.5 mg/kg (Age Group 3: >=6 months and 3 years of age) orally twice daily for 7 days. | Drug: JNJ-53718678<br>* Participants will receive JNJ-53718678 (high dose or low dose) orally twice daily for 7 days.<br>|
| Placebo Comparator: Placebo<br>Participants will be randomized to receive matching placebo (i.e. high volume placebo or low volume placebo to match the calculated volume of the JNJ-53718678 for the high dose or low dose) orally twice daily for 7 days. | Drug: Placebo<br>* Participants will receive matching placebo (high volume or low volume) orally twice daily for 7 days.<br>|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Respiratory Syncytial Virus (RSV) Viral Load Area Under Curve (AUC) From Immediately Prior to First Dose of Study Drug (Baseline) Through Day 5 (AUC[Day 5]) | 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. | Baseline through Day 5 | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| RSV Viral Load Over Time | RSV viral load actual values over time were 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. | Baseline, Days 3, 5, 8, 14, and 21 | 
| Change From Baseline in RSV Viral Load Over Time | 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. | Baseline up to Days 3, 5, 8, 14, and 21 | 
| Least Squares (LS) Mean RSV Viral Load on Days 3, 8 and 14 | 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. | Baseline through Days 3, 8, and 14 | 
| Time to Undetectable RSV Viral Load | Time to undetectable RSV viral load (as measured by qRT-PCR) was defined as the time in hours from first dose of study drug to first post-baseline timepoint at which the virus was undetectable and after which there were no more detectable virus assessments. As planned, combined data for both the cohorts was collected, analyzed and reported for this outcome measure. | Up to Day 21 | 
| Percentage of Participants With Undetectable RSV Viral Load at Each Timepoint Throughout the Study | 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. | Baseline, Days 3, 5, 8, 14 and 21 | 
| Change From Baseline in Parent(s)/Caregiver(s) Pediatric RSV Electronic Severity and Outcomes Rating System (PRESORS) Scores | 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. | Baseline up to Days 3, 5, 8, 14 and 21 | 
| Change From Baseline in Clinician PRESORS Score | 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. | Baseline, up to Days 3, 5, 8, 14 and 21 | 
| Time to Resolution of RSV Symptoms Based on PRESORS Caregiver (ObsRO) | Time to resolution is defined as time from first dose of study drug until the first time of resolution of all RSV symptoms (breathing problems, retractions, tachypnea, breathing sounds, cough, tachycardia, nasal secretions, sleep disturbance, crying, illness behavior, feeding problems, and dehydration). Resolution occurs when all symptoms from the caregiver reported outcomes (ObsRO) are scored as none or mild (score of 0 or 1, respectively) for at least 24 hours. | Up to Day 21 | 
| Time to Improvement on Overall Health | Time to improvement based on general questions on overall health was assessed. Time from first dose of study drug until first time status of improvement of RSV symptoms reported as very much improved or much improved based on response to 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'. | Up to Day 21 | 
| Percentage of Participants by Status of RSV Symptoms Based on PRESORS Caregiver (ObsRO) General Question Over Time | 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. | Baseline, Days 3, 5, 8, 14, and 21 | 
| Percentage of Participants by Health Status Assessment Based on PRESORS Caregiver (ObsRO) General Question Over Time | 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. | Baseline, Days 3, 5, 8, 14, and 21 | 
| Percentage of Participants With Worsening or Improvement Status of RSV Disease | 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. | Days 14 and 21 | 
| Percentage of Participants by Return to Pre-RSV Disease Health Status Assessment Based on PRESORS Caregiver (ObsRO) General Question Over Time | 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'. | Baseline, Days 3, 5, 8, 14, and 21 | 
| Respiratory Rate (RR) Over Time | Respiratory rate (RR) was measured by the investigator over time. | Baseline, Days 3, 5, 8, 14 and 21 | 
| Change From Baseline in Respiratory Rate | 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. | Baseline to Days 3, 5, 8, 14 and 21 | 
| Heart Rate Over Time | Heart rate was measured by the investigator over time. | Baseline, Days 3, 5, 8, 14 and 21 | 
| Change From Baseline in Heart Rate | Change from baseline in heart rate was assessed. | Baseline to Days 3, 5, 8, 14 and 21 | 
| Body Temperature Over Time | Body temperature was reported over time (either investigator or caregiver measured). | Baseline, Days 3, 5, 8, 14 and 21 | 
| Change From Baseline in Body Temperature | Change from baseline in body temperature (either investigator or caregiver measured) was assessed. | Baseline to Days 3, 5, 8, 14 and 21 | 
| Peripheral Capillary Oxygen Saturation (SpO2) Over Time | Peripheral capillary oxygen saturation was measured by the investigator over time. | Baseline, Days 3, 5, 8, 14 and 21 | 
| Change From Baseline in Peripheral Capillary Oxygen Saturation (SpO2) | Change from baseline in peripheral capillary oxygen saturation levels was derived based on reported values over time. | Baseline to Days 3, 5, 8, 14, and 21 | 
| Percentage of Participants Who Required (re)Hospitalization During Treatment and Follow-up | 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). | Up to Day 28 | 
| Cohort 1: Time to Return to Age-adjusted Normal Values for Vital Signs | 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. | Up to Day 28 | 
| Cohort 1: Time to Discharge From Hospital | 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. | Up to Day 28 | 
| Cohort 1: Percentage of Participants Who Required Admission to the Intensive Care Unit (ICU) | 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. | Up to Day 21 | 
| Cohort 1: Duration of ICU Stay | 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. | Up to Day 21 | 
| Cohort 1: Time to Clinical Stability Evaluated by the Investigator | 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. | Up to Day 21 | 
| Cohort 1: Percentage of Participants Who Required Supplemental Oxygen | 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. | Up to Day 21 | 
| Cohort 1: Duration of Supplemental Oxygen | 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. | Up to Day 28 | 
| Cohort 1: Percentage of Participants Who Required Non-invasive Mechanical Ventilation Support | 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. | Up to Day 21 | 
| Cohort 1: Percentage of Participants Who Required Invasive Mechanical Ventilation Support | 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. | Up to Day 21 | 
| Cohort 1: Percentage of Participants Who Required Non-invasive Non-mechanical Ventilation Support | 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. | Up to Day 21 | 
| Cohort 1: Duration of Non-invasive Ventilation Support | 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. | Up to Day 21 | 
| Cohort 1: Duration of Invasive Ventilation Support | 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. | Up to Day 21 | 
| Cohort 1: Time to End of Supplemental Oxygen up to 72 Hours From First Hospital Discharge | 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. | Up to end of supplemental oxygen including supplemental oxygen within 72 hours after first hospital discharge (up to Day 28) | 
| Cohort 1: Percentage of Participants Who Needed Hydration and/or Feeding by Intravenous (IV) Administration or Nasogastric Tube | 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. | Up to Day 28 | 
| Percentage of Participants With Adverse Events | 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. | Up to Day 28 | 
| Percentage of Participants With Abnormal Laboratory Findings | 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. | Up to Day 28 | 
| Percentage of Participants With Abnormal Electrocardiograms (ECGs) Findings | 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. | Up to Day 28 | 
| Percentage of Participants With Categorized Change From Baseline in ECG Parameters (QT, QTcB, QTcF) | 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. | Baseline to Day 28 | 
| Percentage of Participants With Vital Signs Abnormalities | 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. | Up to Day 28 | 
| Cohort 1: Area Under the Plasma Concentration-Time Curve From Timepoint 0 Hours Until 24 Hours Post Dose (AUC[0-24 Hours]) | 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. | 0 to 24 hours post dose on Days 1 and 7 | 
| Cohort 1: Maximum Plasma Concentration (Cmax) of JNJ-53718678 | Cmax is the maximum plasma concentration of JNJ-53718678 estimated by population PK model. | Days 1 and 7 | 
| Cohort 1: Trough Plasma Concentration (Ctrough) of JNJ-53718678 | Ctrough is the trough plasma concentration of JNJ-53718678 estimated by population PK model. | Days 1 and 7 | 
| Cohort 2: Plasma Concentration of JNJ-53718678 | 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. | 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 | 
| Percentage of Participants With Medical Resource Utilization (MRU) | Percentage of participants with MRU (any medical care encounters) was reported. | Up to Day 28 | 
| Percentage of Participants With Acceptability and Palatability of the JNJ-53718678 Formulation as Assessed by Parent(s)/Caregiver(s) | 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. | Day 8 | 
| Number of Participants With Emerging Variations in the Viral Genome Potentially Associated With Resistance to JNJ-53718678 | 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. | Up to Day 21 |