via @sammyj98 via UpToDate:
Universal prophylaxis with
ganciclovir is typically given to patients at risk for cytomegalovirus (CMV) reactivation (eg, seropositive recipients and those with seropositive donors). The duration of therapy often depends on the type of organ transplanted, the risk status of the patient, and individual institutional practice. Some transplant centers prefer to use a pre-emptive approach (eg, routine CMV viral load monitoring within initiation of treatment when reactivation becomes evident) for specific patient populations.