A Study to Evaluate the Safety and Tolerability, Pharmacokinetics, and Antiviral Activity of Maribavir for the Treatment of Cytomegalovirus (CMV) Infection in Children and Adolescents Who Have Received a Hematopoietic Stem Cell Transplant (HSCT) or a Solid Organ Transplant (SOT)

Inclusion Criteria:

• Parent/both parents or legally authorized representative (LAR) must providesignature of informed consent and there must be documentation of assent by theparticipant, as age appropriate, before completing any study-related procedures.

• Be a male or female child or adolescent < 18 years of age at the time of consent.For participants in Cohort 3 only (0 to <6 years) must have a gestational age of atleast 39 weeks and a minimum weight of 5 kg.

• Be a recipient of an SOT or an HSCT that is functioning at the time of screening.

• Have a documented CMV infection which may be a first episode of post-transplant CMVviremia (primary or reactivation) or refractory to other anti-CMV treatments, with aCMV DNA screening value of >= 1365 International Units per milliliter (IU/mL) inwhole blood or >= 455 IU/mL in plasma in 2 consecutive assessments separated by atleast 1 day, as determined by local laboratory quantitative polymerase chainreaction (qPCR) or comparable quantitative nucleic acid amplification test (qNAAT)results. Quantitative assays must be standardized to the World Health Organization (WHO) CMV International Standard. Both samples must be taken within 14 days of firstdose of study drug, with the second sample obtained within 5 days prior to firstdose of study drug. The same laboratory and same sample type (whole blood or plasma)must be used for both assessments. If documented and verified values are availablein medical history that fulfill this criterion entirely, they may be used instead.

• Have all the following results as part of screening laboratory assessments:

• Absolute neutrophil count >= 500 per cubic millimeter (/mm^3) (0.5 × 10^9 per liter [/L])

• Platelet count >= 15,000/mm^3 (15 × 10^9/L)

• Hemoglobin >= 8 grams per deciliter (g/dL) (>=80 grams per liter [g/L]).

• Have an estimated glomerular filtration rate (creatinine-based Bedside Schwartzequation) >= 30 milliliters per minute (mL/min) /1.73 meter square (m^2).

• Be a female of nonchildbearing potential. If a female of childbearing potential,have a negative serum human chorionic gonadotropin (hCG) or beta-human chorionicgonadotropin (β-hCG) pregnancy test at screening. Males, or nonpregnant,nonlactating females who are sexually active must agree to comply with theapplicable contraceptive requirements of this protocol during the study treatmentadministration period and for 90 days after the last dose of study treatment.

• Have life expectancy of >= 8 weeks.

• Be willing and have an understanding and ability to fully comply with the studyprocedures and restrictions defined in the protocol. For younger children, theparent/both parents or LAR must meet this criterion.

• Participants must have a confirmed negative human immunodeficiency virus (HIV) testresult within 3 months of first dose of study drug or, if unavailable, be tested bya local laboratory during the screening period.

Exclusion Criteria:

• Have CMV tissue invasive disease involving the central nervous system (CNS) orretina as assessed by the investigator at the time of screening.

• Have uncontrolled other type of infection as assessed by the investigator on thedate of enrollment.

• Have a history of clinically relevant alcohol or drug abuse that may interfere withtreatment compliance or assessments with the protocol as determined by theinvestigator.

• Be receiving valganciclovir, ganciclovir, cidofovir, foscarnet, leflunomide,letermovir, or artesunate when study treatment is initiated, or anticipated torequire one of these agents during the 8-week treatment period.

• Have a known hypersensitivity to maribavir or to any excipients.

• Have severe vomiting, diarrhea, or other severe gastrointestinal (GI) illness within 24 hours prior to the first dose of study treatment or a GI absorption abnormalitythat would preclude administration of oral medication.

• Require mechanical ventilation or vasopressors for hemodynamic support at baseline (Visit 2/Day 1/Week 0).

• Be pregnant (or expecting to conceive) or nursing.

• Have previously completed, discontinued, or have been withdrawn from this study.

• Have received any investigational agent or device within 30 days before initiationof study treatment (includes CMV specific T-cells) or plan to receive aninvestigational agent or device during the study.

Previously approved agents under investigation for additional indications are not exclusionary.

• Have previously received maribavir or CMV vaccine at any time.

• Have any clinically significant medical or surgical condition that, in theinvestigator's opinion, could interfere with interpretation of study results,contraindicate the administration of the study treatment, or compromise the safetyor well-being of the participant.

• Have severe liver disease (Child-Pugh score of >= 10).

• Have serum aspartate aminotransferase greater than (>) 5 times upper limit of normal (ULN) at screening, or serum alanine aminotransferase > 5 times ULN at screening, ortotal bilirubin >= 3.0 times ULN at screening (except for documented Gilbert'ssyndrome), as analyzed by local laboratory.

• Have positive results for HIV.

• Have active malignancy with the exception of nonmelanoma skin cancer, as determinedby the investigator. Participants who experience relapse or progression of theirunderlying malignancy (for which HSCT or SOT was performed), as determined by theinvestigator, are not to be enrolled.

• Be undergoing treatment for acute or chronic hepatitis B or hepatitis C.

• Requiring ongoing treatment with or an anticipated need for treatment with a strongcytochrome P450 3A (CYP3A) inducer.

• Have a low body weight where total blood volume (TBV) required during studyparticipation will exceed 1 percent (%) TBV per study visit or 3% TBV over a 4-weekperiod.