Efficacy and Safety Study of Maribavir Compared to Valganciclovir for the Treatment of Cytomegalovirus (CMV) Infection in Hematopoietic Stem Cell Transplant Recipients

Inclusion Criteria:

- Be able to provide written, personally signed, and dated informed consent to
participate in the study before completing any study-related procedures. As
applicable, a parent/both parents or legally authorized representative (LAR) must
provide signature of informed consent and there must be documentation of assent by the
participants before completing any study-related procedures.

- Be greater than or equal to (>=) 16 years of age at the time of consent.

- Be a recipient of hematopoietic stem cell transplant.

- Have a documented asymptomatic CMV infection, with a screening value of CMV DNA >=1365
International Units per millilitre (IU/mL) to less than or equal to (<=) 273000 IU/mL
in whole blood or >= 455 IU/mL to <= 91000 IU/mL in plasma in 2 consecutive
assessments, separated by at least 1 day, as determined by local or central specialty
laboratory quantitative polymerase chain reaction (qPCR) or comparable quantitative
CMV DNA results. Both samples should be taken within 14 days prior to randomization
with second sample obtained within 5 days prior to randomization. Same laboratory and
same sample type (whole blood or plasma) should be used for these assessments.
Asymptomatic CMV infection is defined as an infection that does not present with
tissue invasive CMV disease, as assessed by the investigator. Participants with CMV
DNA less than (<) 910 and >= 455 IU/mL in plasma or < 2730 and >= 1365 IU/mL in whole
blood will also need to meet at least 1 of the following criteria for high-risk CMV
infection to be eligible:

1. Human leukocyte antigen (HLA)-related (sibling) donor with at least 1 mismatch at
1 of the following 3 HLA-gene loci: HLA-A, -B or -DR,

2. Haploidentical donor

3. Unrelated donor with at least 1 mismatch at 1 of the following 4 HLA -gene loci:
HLA-A, -B, -C and -DRB1,

4. Use of umbilical cord blood as stem cell source,

5. Use of ex vivo T-cell-depleted grafts,

6. Grade 2 or greater graft-versus-host-disease (GVHD), requiring the use of
systemic corticosteroids (defined as the use of >= 1 milligram per kilogram per
day (mg/kg/day) of prednisone or equivalent dose of another corticosteroid).

- Have the current CMV infection as the first episode of CMV viremia after HSCT, either
primary or reactivation, which in the investigator's opinion requires treatment.

- Per investigator's judgment, be eligible for treatment with valganciclovir.

- Have all of the following results as part of screening laboratory assessments (results
from either the central laboratory or a local laboratory can be used for
qualification):

1. Absolute neutrophil count to >= 1000 per cubic millimeter (/mm^3) [1.0 x 10^9/L].

2. Platelet count >= 25,000/mm^3 [25 x 10^9/L].

3. Hemoglobin >= 8 grams per deciliter (g/dL).

4. Estimated creatinine clearance >=30 milliliters per minute (mL/min).

- Have a negative serum beta human chorionic gonadotropin (beta-HCG) pregnancy test at
screening, if a female of child bearing potential. Urine pregnancy tests may be done
per institutional requirements; however they are not sufficient for eligibility
determination. Sexually active females of child bearing potential must agree to comply
with any applicable contraceptive requirements of the protocol. If male, must agree to
use an acceptable method of birth control, as defined in the protocol, during the
study treatment administration period and for 90 days afterward the last dose of study
treatment.

- Be able to swallow tablets.

- Have life expectancy of >= 8 weeks.

- Weigh >= 40 kilograms (kg).

- Be willing and have an understanding and ability to fully comply with study procedures
and restrictions defined in the protocol.

Exclusion Criteria:

- Have CMV tissue invasive disease as assessed by the investigator at the time of
screening and randomization at Visit 2/Day 0.

- Have a CMV infection that is known to be genotypically resistant to ganciclovir,
valganciclovir, foscarnet, or cidofovir based on documented evidence.

- Be presenting with recurrent CMV infection (defined as a new detection of CMV
infection in a participants who had at least one previously documented episode of CMV
infection post-transplant, and who has had at least 2 weeks of undetectable CMV DNA
between the episodes during active surveillance, based on same local laboratory and
same sample type). The Participants must also have been off any anti-CMV treatment
between the current and prior infection. Otherwise, the current infection may be
considered continuation of the prior infection.

- Require ganciclovir, valganciclovir, foscarnet, or cidofovir administration for
conditions other than CMV when study treatment is initiated (example: herpes simplex
virus [HSV] co-infection requiring use of any of these agents after the randomization)
or would need a co-administration with maribavir for CMV infection.

- Be receiving leflunomide, letermovir, or artesunate when study treatment is initiated.

Note: Participants who may be receiving leflunomide must discontinue the use at least 14
days prior to randomization at Visit 2/Day 0 and the first dose of study treatment.
Participants receiving letermovir must discontinue use 3 days prior to first dose of study
treatment. Participants receiving artesunate must discontinue the use prior to the first
dose of study treatment.

- Be on treatment with anti-CMV agents (ganciclovir, valganciclovir, foscarnet or
cidofovir) for the current CMV infection for longer than 72 hours.

- Have known hypersensitivity to the active substance or to an excipient of the study
treatments.

- Have severe vomiting, diarrhea, or other severe gastrointestinal illness within 24
hours prior to the first dose of study treatment that would preclude administration of
oral medication.

- Require mechanical ventilation or vasopressors for hemodynamic support at the time of
randomization.

- Be female and pregnant or nursing.

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

- Have received any investigational agent with known anti-CMV activity within 30 days
before initiation of study treatment or CMV vaccine at any time.

- Have received any unapproved agent or device within 30 days before initiation of study
treatment.

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

- Have previously received maribavir.

- Have serum aspartate aminotransferase (AST) greater than (>) 5 times upper limit of
normal (ULN) at screening, or serum alanine aminotransferase (ALT) > 5 times ULN at
screening, or total bilirubin >= 3.0 x ULN at screening (except for documented
Gilbert's syndrome), as analyzed by local or central lab.

- Have known (previously documented) positive results for human immunodeficiency virus
(HIV). Participants must have a confirmed negative HIV test result within 3 months of
study entry or, if unavailable, be tested by a local laboratory during the screening
period.

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

- Be undergoing treatment for acute or chronic hepatitis C