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

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.

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

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 weights
<2.4 kilogram (kg) or greater than or equal to (>=) 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 with clinically significant abnormal electrocardiogram findings or
prolonged Corrected QT by Fridericia Formula (QTcF)-interval at screening