Percentage of participants who developed RSV LRTI was assessed. RSV LRTI was defined as the development of a lower respiratory sign or symptom (including decrease in oxygen saturation or increase in supplemental oxygen to maintain oxygen saturation, wheezing, rhonchi, rales, dyspnea, tachypnea, worsening cough) and positive RSV test from lower respiratory tract sample (example [eg], sputum, induced sputum, bronchoalveolar lavage (BAL), lung biopsy, or autopsy specimen) within +-4 days of a new chest image finding, compared to baseline, consistent with a LRTI; OR positive RSV test from lower respiratory tract sample (eg, sputum, induced sputum, BAL, lung biopsy, or autopsy specimen) only; OR positive RSV test from upper respiratory tract sample within ±4 days of a new chest image finding, compared to baseline, consistent with a RSV LRTI as determined by the Endpoint Adjudication Committee (EAC).

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who developed RSV-associated LRTC was assessed. RSV-associated LRTC defined as development of lower respiratory sign/symptom (includes decrease in oxygen saturation/increase in supplemental oxygen to maintain oxygen saturation, wheezing, rhonchi, rales, dyspnea, tachypnea, and worsening cough) and met 1 of following subcategories determined by EAC: a) RSV LRTI, b) secondary bacterial LRTI (positive specimen for clinically significant bacterium within 4 days of new chest image finding, compared to baseline, consistent with LRTI), c) secondary LRTI due to unusual pathogens (positive specimen for clinically significant unusual organism within 4 days of new chest image finding, compared to baseline, consistent with LRTI), d) secondary LRTC of unknown etiology (new chest image finding than baseline, consistent with LRTI, inflammatory process/ some other clinically significant pulmonary process which were absent within 4 days of new chest image finding).

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

An AE is any untoward medical occurrence in a clinical study participant administered a medicinal (investigational or non investigational) product. An AE did not necessarily have a causal relationship with the intervention. Any AE which occurred post first dose administration of study drug up to the end of study (EOS) (that is, Day 49) was considered as treatment-emergent.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Percentage of participants with greater than or equal to (>=) Grade 3 treatment-emergent laboratory abnormalities (platelet count decreased, glucose increase) was assessed in this outcome measure. Treatment-emergent: any abnormality occurred post first dose of study drug up to end of study (that is, Day 49).

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Percentage of participants with clinically significant abnormalities in ECG findings was assessed in this outcome measure. Various ECG variables assessed were heart rate: abnormally low (<= 45 beats per minute [bpm]), abnormally high (>= 120 bpm); PR interval: abnormally high (>=210 milliseconds [msec]); QRS interval: abnormally high (>=120 msec), QT interval and corrected QT (QTcF; according to Fridericia's formula) interval (>450 msec, >480 msec, or >500 msec, increases from baseline >30 msec or >60 msec.)

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Percentage of participants with abnormal vital signs findings was assessed. Abnormal vital parameters included pulse rate: abnormally low <=45 bpm, abnormally high >=120 bpm; Systolic Blood Pressure (SBP): abnormally low <=90 Millimeter of mercury (mmHg), Grade 1 (mild): > 90 mmHg - < 100 mmHg, Grade 2 (moderate): >= 100 mmHg to <110 mmHg, Grade 3 (severe): >=110 mmHg; Diastolic BP: abnormally low <=50 mmHg, Grade 1: >90 mmHg to <100 mmHg, Grade 2: >=100 mmHg to <110 mmHg, Grade 3: >=110 mmHg; Respiratory rate- Grade 1 (mild): 17-20 breaths per minute, Grade 2 (moderate): 21-25 breaths per minute, Grade 3 (severe): >25 breaths per minute, Grade 4 (potentially life threatening): intubation; Temperature: abnormally high >38.0 degree celsius. Vital signs abnormalities reported for at least 1 participant were reported in this outcome measure. Treatment-emergent: any abnormality occurred post first dose of study drug up to EOS (that is, Day 49).

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Percentage of participants who progressed to respiratory failure (of any cause) requiring mechanical ventilation (invasive or noninvasive) and/or death among those who developed RSV LRTI or RSV-associated LRTC per the EAC's assessment was assessed. Here, '0' in the 'number of participants analyzed' field (N=0) signifies that no participants were available for the analysis because none of the participants developed RSV LRTI or RSV-associated LRTC per the EAC's Assessment.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who progressed to respiratory failure (of any cause) requiring mechanical ventilation (invasive or noninvasive) and/or death (all-cause mortality) was assessed.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who progressed to death (all-cause mortality) among those who developed RSV LRTI or RSV-associated LRTC per the EAC's assessment was assessed. Here, '0' in the 'number of participants analyzed' field (N=0) signifies that no participants were available for the analysis because none of the participants developed RSV LRTI or RSV-associated LRTC per the EAC's Assessment.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who progressed to death (all-cause mortality) was assessed.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who progressed to respiratory failure (of any cause) requiring mechanical ventilation (invasive or noninvasive) among those who developed RSV LRTI or RSV-associated LRTC per the EAC's assessment was assessed. Here, '0' in the 'number of participants analyzed' field (N=0) signifies that no participants were available for the analysis because none of the participants developed RSV LRTI or RSV-associated LRTC per the EAC's Assessment.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who progressed to respiratory failure (of any cause) requiring mechanical ventilation (invasive or noninvasive) was assessed.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Number of supplemental oxygen free days was assessed. The number of supplemental oxygen free days was the number of days the participants did not receive/require supplemental oxygen during the first 28 days post treatment.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized. Here, "n" (number analyzed)" is defined as number of participants analyzed for each specified category.

Percentage of participants who required treatment-emergent oxygen supplementation (e.g., supplemental oxygen, noninvasive pressure ventilation, invasive mechanical ventilation [tracheal tube, laryngeal mask or tracheostomy]). Any AE which occurred post first dose administration of study drug up to the end of study (that is, Day 49) were considered as treatment-emergent.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, and were analyzed as randomized.

Respiratory rate over time was reported by investigator. In this outcome measure, only those timepoints in which individual participant had data were reported.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure and 'n' (number analyzed) represent number of participants with available data for each specified timepoint.

Heart rate over time was reported by investigator. In this outcome measure, only those timepoints in which individual participant had data were reported.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'. Here, 'n' (number analyzed) represent number of participants with available data for each specified timepoint.

Peripheral capillary oxygen saturation (SpO2) over time was reported by investigator. In this outcome measure, only those timepoints in which individual participant had data were reported.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'. Here, 'n' (number analyzed) represent number of participants with available data for each specified timepoint.

Body temperature (in Degrees Celsius) over time was reported. In this outcome measure, only those timepoints in which individual participant had data were reported.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'. Here, 'n' (number analyzed) represent number of participants with available data for each specified timepoint.

Percentage of participants who were not hospitalized at baseline and required hospitalization during the study was assessed.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Percentage of participants who were re-hospitalized (of participants who were hospitalized at baseline and discharged during the study and of participants who were not hospitalized at baseline and required hospitalization and were discharged during the study) was assessed in this outcome measure.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Duration (in days) of hospital stay was assessed.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized. Here, 'N' (number of participants analyzed) signifies number of participants who were evaluable for this outcome measure.

Duration of ICU stay was assessed. Duration (in hours) was defined as total number of hours a participant was in ICU from first dose of study drug until study termination.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

An AE is any untoward medical occurrence in a clinical study participant administered a medicinal (investigational or non investigational) product. An AE did not necessarily had a causal relationship with the intervention. Participants with Grade 3 or Grade 4 TEAE were assessed in this outcome measure. Any AE which occurred post first dose administration of study drug up to the end of study (that is, Day 49) were considered as treatment-emergent.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Number of participants with respiratory related AEs (respiratory infections) was assessed.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Number of participants with thoracic-related AEs was assessed.

The safety analysis set included all randomized participants who took at least 1 dose of study intervention and were analyzed 'as treated'.

Number of participants with antibiotic use among those who developed RSV LRTI or RSV-associated LRTC per the EAC's assessment was assessed. Here, '0' in the 'number of participants analyzed' field (N=0) signifies that no participants were available for the analysis because none of the participants developed RSV LRTI or RSV-associated LRTC per the EAC's Assessment.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized.

Plasma Concentration of JNJ-53718678 was reported. In this outcome measure, only those timepoints in which individual participant had data were reported.

PK analysis set included all participants who received JNJ-53718678 and for whom at least one PK concentration was reported. Here, 'n' (number analyzed) represents number of participants with available data for each specified timepoint.

RSV viral load (RSV B) was measured over time by quantitative reverse transcription polymerase chain reaction (qRT-PCR) in the nasal swab specimens collected at the clinic visits and at home. In this outcome measure, only those timepoints in which individual participant had data were reported.

Efficacy analysis set included all participants who were randomized, treated (took at least 1 dose), and had a RSV infection confirmed by central laboratory analysis, excluding participants infected with a co-pathogen at baseline not identified during screening, analyzed as randomized. Here, 'n' (number analyzed) represents number of participants with available data for each specified timepoint.