A Study of Maribavir Compared to Valganciclovir to Treat Cytomegalovirus Infections in People Who Have Received Stem Cell Transplants

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. During the COVID-19 public health emergency, informed consent from a potential or current trial participant may, if permitted by local laws and regulations, be obtained via electronic informed consent (eIC) capabilities or an electronic face-to-face consent interview when these individuals are unable to travel to the site (FDA COVID-19 Guidance, 27 January 2021, Q11).
• 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 ( =455 IU/mL to =455 IU/mL in plasma or =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:
• 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,
• Haploidentical donor
• Unrelated donor with at least 1 mismatch at 1 of the following 4 HLA -gene loci: HLA-A, -B, -C and -DRB1,
• Use of umbilical cord blood as stem cell source,
• Use of ex vivo T-cell-depleted grafts,
• 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):
• Absolute neutrophil count to >=1000 per cubic millimeter (/mm^3) [1.0*10^9/L].
• Platelet count >=25,000/mm^3 [25*10^9/L].
• Hemoglobin >=8 grams per deciliter (g/dL).
• 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