Frequent human herpesvirus-6 viremia but low incidence of encephalitis in double-unit cord blood recipients transplanted without antithymocyte globulin.

Cord blood transplantation (CBT) is a known risk factor for human herpesvirus-6 (HHV-6) infection. We analyzed the nature of HHV-6 infections in 125 double-unit CBT recipients (median age, 42 years) transplanted for hematologic malignancies with calcineurin inhibitor/mycophenolate mofetil prophylaxis and no antithymocyte globulin. One hundred seventeen patients (94%) reactivated HHV-6 by quantitative plasma PCR (median peak, 7600 copies/mL; range, 100 to 160,000) at a median of 20 days (range, 10 to 59) after transplantation. HHV-6 encephalitis occurred in 2 patients (1.6%), of whom 1 died and 1 recovered with therapy. No association was found between high-level HHV-6 viremia (≥10,000 or ≥25,000 copies/mL) and age, diagnosis, conditioning intensity, or dominant unit characteristics or between high-level viremia and transplant outcomes (engraftment, cytomegalovirus reactivation, day 100 grades II to IV acute graft-versus-host disease, day 100 transplant-related mortality, or 1-year disease-free survival). HHV-6 therapy delayed the onset of cytomegalovirus reactivation. Interestingly, HHV-6 resolution was observed in untreated patients, and resolution of viremia correlated with absolute lymphocyte count recovery. We observed a low incidence of encephalitis and no association with CBT outcomes. Our data suggest therapy in uncomplicated viremia may not be warranted. However, further investigation of the risk-to-benefit of HHV-6 viremia treatment and standardization of PCR testing is required.