This is a Phase 1b, randomized (study medication assigned to participants by chance), partially double-blind (neither physician nor participant knows the identity of the assigned treatment), placebo-controlled, multicenter, multiple ascending dose study of JNJ 53718678 in infants (greater than [>] 1 month to less than or equal to [<=] 24 months of age) who are hospitalized with RSV infection. The duration of study will be approximately 4 weeks for each participant excluding screening period. In Part 1 of study, minimum total number of 42 participants will be divided in 3 cohorts: Age group 1 (Cohorts 1a-1e) (greater than or equal to [>=] 6 months and less than or equal to [<=] 24 months of age), Age group 2 (Cohorts 2a-2e)(>=3 months and less than [<] 6 months of age) and Age group 3 (Cohorts 3a-3e) (greater than [>] 1 month and <3 months of age). Each age group will consist of a minimum of 3 cohorts with the possibility to add 2 more per age group (Cohorts a through e) in which different doses and/or dosing regimens will be evaluated. Each cohort will consist of 5 participants (4 participants receiving JNJ-53718678 and 1 participant receiving placebo for 7 days), except for the first cohort of each age group which will contain only 4 participants (4 participants receiving JNJ 53718678). In Part 2 of the study, all age groups will be included in a single cohort, Cohort f, in which the selected dose regimen determined during Part 1 of the study will be used for each of the 3 age groups. A minimum of approximately 18 (12 participants receiving JNJ 53718678 and 6 participants receiving placebo) and a maximum of 24 participants (16 participants receiving JNJ 53718678 and 8 participants receiving placebo) will be included in this part of the study. Pharmacokinetics and safety of JNJ-53718678 will be evaluated primarily. Participants' safety will be monitored throughout the study.
The purpose of this study is to evaluate pharmacokinetics, safety, tolerability, antiviral activity, and impact on the clinical course of Respiratory Syncytial Virus (RSV) infection after multiple oral doses of JNJ-53718678 at different doses and/or dosing regimens in infants (greater than [>] 1 month to less than or equal to [<=] 24 months of age) who are hospitalized with RSV infection.