Explore the tolerability and efficacy of letermovir in the prevention of CMV reactivation in patients with acute and chronic graft-versus-host disease (GVHD) beyond day 100.

42 patients meeting the inclusion criteria will receive letermovir 480 mg per day. Patients receiving concomitant cyclosporine will receive 240 mg of letermovir. Letermovir will be given during 14 weeks or until prednisone dose is reduced below 0.2 mg / kg / day, whichever comes earlier.

All the patients will continue herpesvirus prophylaxis with acyclovir or valacyclovir.

Patients will be evaluated biweekly until letermovir discontinuation. A PCR CMV test will be performed biweekly. Patients in whom clinically significant CMV infection (defined as CMV disease or CMV viremia leading to preemptive treatment) develop, will discontinue the trial regimen and begin anti-CMV therapy according to local practice. The threshold for preemptive treatment for CMV viremia will be >1000 copies/ ml.

Inclusion Criteria:

• Adult patients
• After allogeneic transplantation
• Seropositive for CMV
• Who already received letermovir prophylaxis until day 100 without CMV reactivation
• Beyond day 100 after transplantation at enrollment
• With acute or chronic GVHD
• Daily prednisone dose ≥ 0.5 mg/kg (or equivalent dose of another glucocorticoid) at enrollment.
• Negative polymerase chain reaction (PCR) for CMV at enrollment
• Provided informed consent

Exclusion Criteria:

• Seronegative for CMV
• Positive PCR for CMV at enrollment
• Patient has previously had CMV reactivation under letermovir.