A Study of the Drug Letermovir as Prevention of Cytomegalovirus Infection After Stem Cell Transplant in Pediatric Patients

Inclusion Criteria: * \>= 2 years and \< 18 years at the time of enrollment * Weight must be \>= 6 kg at the time of enrollment * Planned allogeneic HCT (bone marrow, peripheral blood stem cell, or cord blood transplant) * Patient must be CMV sero-positive (i.e., recipient CMV immunoglobulin G positive) * Note: If a patient has hypogammaglobulinemia but has previously been documented as CMV sero-positive, that is acceptable for study inclusion. For all patients already confirmed to be CMV IgG seropositive, repeat testing is not required within 7 days prior to enrollment. However, the laboratory data determining eligibility must be available in the patient's medical/research record for verification * Patient is eligible for entry only if it is feasible for plasma CMV PCR testing to be sent and resulted within the protocol mandated time period * Reminder: To limit the likelihood of positive plasma CMV PCR post-enrollment and prior to start of study treatment period, it is recommended that patient enrollment proceed after patients start their transplant preparative regimen * Patient must have a performance status corresponding to Lansky/Karnofsky scores \> 50 * Note: Use Lansky for patients =\< 16 years of age and Karnofsky for patients \> 16 years of age. For further reference, see performance status scales scoring under the standard sections for protocols among protocol reference materials provided on the Children's Oncology Group (COG) member website: https://member…