Letermovir is approved for the primary prevention of Cytomegalovirus (CMV) reactivation and infection in hematopoietic stem cell transplant recipients. Letermovir may be beneficial in other clinical presentation where CMV reactivates and may alter clinical outcomes. Recently Chimeric Antigen Receptor (CAR) T cells have been used for the treatment of refractory acute leukemia and B cell lymphoma. Reactivation of chronic viral infections, in particular those belonging to the Herpesviridae family can therefore be observed following CAR-T cells treatment.According to first reports, Cytomegalovirus seems to be the main virus detected. Uncontrolled CMV reactivation leads to CMV disease requiring the use of antiviral drugs associated with either hematological toxicity (ganciclovir) or renal toxicity (foscarnet) and is usually associated with poor outcomes. In addition, CMV interplays with the immune system and decreases the immunosurveillance of tumor cells and facilitates the growth or reactivation of other opportunistic infections. Therefore, CMV reactivation could also impact the outcome of CART cells treatment by increasing the existing risk of opportunistic infections in CART cells recipients and thus by increasing morbidity, length stay or require intensive care. Imbalance of the immune system usually correlates with reactivation of persistent virus like Torquetenovirus (TTV), redondovirus or pegivirus found more frequently in Hematopoietic stem-cell transplantation (HSCT) patients or patients requiring intensive care. Whether reactivations of those persistent viruses are associated or precede CMV reactivation deserve careful investigation to identify as early as possible patients at high risk and who could benefit from antiviral preventive treatment. The objective of this trial is to determine the incidence of CMV reactivation within 3 months after infusion of CAR-T cells in CMV seropositive patients with refractory acute leukemia or B-cell lymphoma.