Study to Evaluate the Antiviral Activity, Clinical Outcomes, Safety, Tolerability, and Pharmacokinetics of Orally Administered Lumicitabine Regimens in Adult Participants Hospitalized With Respiratory Syncytial Virus

The purpose of this study is to characterize the Pharmacokinetic and to confirm the popPK model derived from healthy volunteers in hospitalized adults who are infected with respiratory syncytial virus (RSV) and to determine in adults who are hospitalized with respiratory syncytial virus (RSV) infection 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) assay.

The study will be conducted in 3 phases: a screening phase, a treatment phase from Day 1 to Day 5/6 (depending on the timing of the loading dose), and a follow-up phase for a total of 28 days post randomization. Participants will have assessments completed at Day 7, Day 10, Day 14, and Day 28. Depending on discharge date, assessments will be completed either while hospitalized or during outpatient visits. The duration of the participant's participation will be approximately 28 days. The study will be performed in 2 parts. Participants will be randomly assigned to one of 2 treatment groups in part 1, and to one of 3 treatment groups in part 2. Treatment groups will be evaluated for PK and safety after a target of approximately 24 participants have been enrolled in part 1 and before initiating part 2 (approximately 90 participants in part 2). An Independent Data Monitoring Committee (IDMC) will be established to monitor the safety of participants and will review data in an unblinded manner on a regular basis to ensure the continuing safety of the participants enrolled in this study and to evaluate whether efficacy objectives are met. The committee will meet periodically to review interim data. Based on the recommendations of the IDMC following interim analyses/reviews, an increase in duration may be implemented.

Eligibility Criteria:
Inclusion Criteria:

* Hospitalized (or in emergency room prior to hospitalization) at the time of randomization and unlikely to be discharged for the first 24 hours after randomization
* Diagnosed with respiratory syncytial virus (RSV) infection based on polymerase chain reaction (PCR)-based assay with or without co infection with another respiratory pathogen (eg, influenza, human metapneumovirus, or bacteria)
* With the exception of the RSV disease, medically stable on the basis of medical history, physical examination, vital signs, and 12-lead electrocardiogram (ECG) performed at screening. If there are abnormalities, they must be consistent with the underlying illness 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
* A woman must have a negative urine beta human chorionic gonadotropin at screening
* A woman must agree not to donate eggs (ova, oocytes) during the study and for at least 44 days after receiving the last dose of study drug
* Contraceptive use by women should be consistent with local regulations regarding the use of contraceptive methods for participants participating in clinical studies A woman must be of non-childbearing potential defined as either: a) Postmenopausal: a postmenopausal state is defined as more than (\>) 45 years and no menses for 12 consecutive months without an alternative medical cause, OR Permanently sterile: permanent sterilization methods include hysterectomy, bilateral salpingectomy, bilateral tubal occlusion/ligation procedures (without reversal operation), and bilateral oophorectomy. b) Of childbearing potential and, if heterosexually active, also included: practicing a highly effective method of contraception (failure rate of less than (\<) 1percent (%) per year when used consistently and correctly)
* Participants must have a body weight of at least 50.0 kilogram, at screening

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 are considered by the investigator to be immuno-compromised within the past 12 months, whether due to underlying medical condition (example, malignancy or genetic disorder) or medical therapy (example, medications other than corticosteroids for the treatment of chronic obstructive pulmonary disease (COPD) or asthma exacerbations, chemotherapy, radiation, stem cell or solid organ transplant)
* Participants with a known history of human immunodeficiency virus (HIV) or chronic viral hepatitis
* Participants undergoing peritoneal dialysis, hemodialysis, or hemofiltration or with an estimated glomerular filtration rate (GFR, determined by Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] equation) of (\<) 60 milliliters per minute (mL/min) per 1.73 meter square (m\^2)
* Participants with 1 or more of the following laboratory abnormalities at screening as defined by the Division of Microbiology and Infectious Diseases (DMID) Adult Toxicity Table: Hemoglobin \<9.5 gram per deciliter (g/dL), Platelet count \<75,000 per millimeter cube (/mm\^³), White blood cell count \<1,000/mm\^³, Absolute neutrophil count \<1,000/mm\^³