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

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 \[Cohort 2\]).

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.

Eligibility Criteria:
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