Transfusion within 24 hours of admission was identified as a risk factor for high-grade CMV viremia in critically ill patients [11] . This association may be explained by potential transmission of CMV by blood products, but more likely by the immunomodulatory effect of transfusion per se. Previous studies have shown that allogeneic blood transfusion resulted in a reduction in T-helper cells, induction of suppressor T cells, and suppression of natural killer cell activity [49] . The transfusionrelated immunosuppression has been associated with clinically important sequelae such as improvement of renal allograft survival, increased risk of tumor recurrence, and postoperative infections [50] [51] [52] . The risk of CMV transmission by leukocyte depleted blood products is at least as low as by CMV seronegative blood products [53, 54] , supporting the hypothesis that transfusion-related immunomodulatory effect plays a major role in CMV infection in critically ill patients if transfusion were truly a risk factor. However, these data were not available for most studies. For example, only one in three studies that evaluated transfusion as potential risk factors for CMV infection reported use of leukocyte depleted blood products explicitly [18] .