An open-label randomized controlled trial to adapt the duration of antiviral prophylaxis in D+/R- patients and in R+ patients receiving ATG according to the result of the T-Track® CMV assay. Investigators will include kidney and liver transplant recipients. Patients will be randomized during the first month post transplant.

In the interventional arm, while the patient is on antiviral prophylaxis, CMI will be monitored every 4 weeks from the 2nd month after transplant. Measurement of CMV CMI will be done in real time by using the T-Track® CMV assay. The continuation of antiviral prophylaxis will depend on the result of the assay:

• T-Track positive (patient at lower risk): discontinuation of the antiviral drug
• T-Track negative (patient at higher risk): continuation of the antiviral drug until the maximal duration of prophylaxis (3 to 6 months) The standard arm will receive a standard fixed duration of antiviral prophylaxis (3 months for R+ thymoglobulin treated-patients and 6 months for D+/R- patients). Measurement of CMV CMI by both T-Track® CMV and the Quantiferon-CMV® assays will be done at the same time points of the interventional arm, but the result will not be known by the investigators.

After discontinuation of prophylaxis, patients in both arms will be followed for the development of CMV replication at each visit using the local PCR assay and antiviral therapy will be administered in case of CMV infection according to local guidelines.

The co-primary endpoints will be the incidence of CMV disease or antiviral-treated CMV replication during the first 12 months post transplant AND the duration of antiviral prophylaxis.

Inclusion Criteria:

• 18 years old
• Kidney or liver transplantation
• Scheduled to receive CMV antiviral prophylaxis:
• CMV D+/R- patients
• Patients receiving lymphocyte-depleting antibodies (thymoglobulin® or ATG®)

Exclusion Criteria:

• Unable to provide informed consent
• Unable or unwilling to comply with study protocol