A Study to Evaluate the Antiviral Activity, Clinical Outcomes, Safety, Tolerability, and Pharmacokinetics of Orally Administered Lumicitabine (JNJ-64041575) Regimens in Hospitalized Infants and Children Aged 28 Days to 36 Months Infected With Respiratory Syncytial Virus

The purpose of this study is to determine in hospitalized infants and children who are infected with respiratory syncytial virus (RSV) the dose-response relationship of multiple regimens of lumicitabine on antiviral activity based on nasal RSV shedding using quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR).

RSV is a leading cause of lower respiratory tract disease in infants. Most infants and children who get RSV recover fully after 1-2 weeks, but RSV infection can sometimes worsen and may lead to hospitalization and admission into an intensive care unit. The main purpose of this study is to learn how well the study drug (lumicitabine, also known as JNJ-64041575 or ALS-008176) works, how the human body handles the study drug, which dose of the study drug is effective for treatment of RSV infection in infants/children and how safe it is compared to a placebo (placebo looks just like lumicitabine \[given in same way\] but has no effect against RSV). Approximately up to 180 participants aged between 28 days to 36 months and hospitalized with RSV infection will take part in this world-wide study.

Eligibility Criteria:
Inclusion Criteria:

* Participants hospitalized (or in emergency room \[ER\]) at the time of randomization and unlikely to be discharged for the first 24 hours after randomization
* Participants diagnosed with respiratory syncytial virus (RSV) infection using a polymerase chain reaction (PCR)-based molecular diagnostic assay, with or without co-infection with another respiratory pathogen (respiratory virus or bacteria)
* Participants who have an acute respiratory illness with signs and symptoms consistent with a viral infection (for example, fever, cough, nasal congestion, runny nose, sore throat, myalgia, lethargy, shortness of breath, or wheezing) with onset less than or equal to \<=5 days from the anticipated time of randomization. Onset of symptoms is defined as the first time (within 1 hour) the parent(s)/caregiver(s) becomes aware of respiratory or systemic symptoms of RSV infection
* With the exception of the symptoms related to the RSV infection or defined comorbid condition for severe RSV disease (prematurity at birth \[participant's gestational age was less than {\<}37 weeks; for infants \<1 year old at randomization\], bronchopulmonary dysplasia, congenital heart disease, other congenital diseases, Down syndrome, neuromuscular impairment, or cystic fibrosis), participant must be medically stable on the basis of physical examination, medical history, vital signs/peripheral capillary oxygen saturation (SpO2), and electrocardiogram (ECG) performed at screening. If there are abnormalities, they must be consistent with the underlying condition in the study population and/or the RSV infection. This determination must be recorded in the participant's source documents and initialed by the investigator. Participants with comorbidities will be allowed to be enrolled once the Independent Data Monitoring Committee (IDMC) has reviewed the pharmacokinetic (PK) and safety data of the highest dose that will be used in this study and once the IDMC has recommended opening recruitment to this group. Sites will be notified when the restriction is lifted
* The participant's estimated glomerular filtration rate (eGFR) is not below the lower limit of normal for the participant's age

Exclusion Criteria:

* Participants who are not expected to survive for more than 48 hours
* Participants who have had major thoracic or abdominal surgery in the 6 weeks prior to randomization
* Participants who have a known or suspected immunodeficiency (except immunoglobulin A \[IgA\] deficiency), such as a known human immunodeficiency virus infection
* Participants being treated with extracorporeal membrane oxygenation
* Participant receiving chronic oxygen therapy at home prior to admission
* Participants who have a poorly functioning gastrointestinal tract (that is, unable to absorb drugs or nutrition via enteral route)