• Parent/both parents or legally authorized representative (LAR) must provide signature of informed consent and there must be documentation of assent by the participant, 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 at least 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 CMV viremia (primary or reactivation) or refractory to other anti-CMV treatments, with a CMV DNA screening value of \>= 1365 International Units per milliliter (IU/mL) in whole blood or \>= 455 IU/mL in plasma in 2 consecutive assessments separated by at least 1 day, as determined by local laboratory quantitative polymerase chain reaction (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 first dose of study drug, with the second sample obtained within 5 days prior to first dose 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 available in 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 Schwartz equation) \>= 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 chorionic gonadotropin (β-hCG) pregnancy test at screening. Males, or nonpregnant, nonlactating females who are sexually active must agree to comply with the applicable contraceptive requirements of this protocol during the study treatment administration 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 study procedures and restrictions defined in the protocol. For younger children, the parent/both parents or LAR must meet this criterion.

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