Dynamics of Varicella Zoster Virus-Specific Immune Reconstitution and Impact of Varicella Vaccination in Pediatric Allogeneic Hematopoietic Stem Cell Transplant Recipients

Background: Varicella-zoster virus infection poses a significant risk to pediatric hematopoietic stem cell transplant recipients owing to compromised T-cell-mediated immunity.Although varicella-zoster virus-specific T-cell-mediated immunity is crucial for preventing reactivation, its reconstitution following allogeneic hematopoietic stem cell transplant in pediatric patients remains poorly characterized.Methods: This prospective study assessed varicella-zoster virus-specific T-cell-mediated immunity in 32 pediatric allogeneic hematopoietic stem cell transplant recipients pretransplant and 1 and 3 months post-transplant using enzyme-linked immune absorbent spot assays.An extension study assessed varicella-zoster virus-specific T-cell-mediated immunity 2 years post-transplant in 15 recipients, including 8 after a single dose of varicella vaccination.Results: Among the 32 recipients (median age, 10.5 years), only one experienced posthematopoietic stem cell transplant varicella-zoster virus infection.Median varicella-zoster virus-specific spot-forming cells were low during the first 3 months but increased significantly at 2 years (0 vs. 5.0/2.0 × 10 5 peripheral blood mononuclear cells; P = 0.005), more evident in recipients with pre-hematopoietic stem cell transplant varicella-zoster virus R + serostatus (P = 0.006) and no prior varicella-zoster virus infection (P < 0.001).The presence of varicellazoster virus-specific T-cell-mediated immunity (≥ 1.0 spot-forming cells/2.0 × 10 5 peripheral blood mononuclear cells) did not differ significantly by pre-hematopoietic stem cell transplant T-cell-mediated immunity status, serostatus, or vaccination history.Among seven recipients assessed at all five time points, recovery-level responses (≥ 5.0 spot-forming cells) increased from 28.6% pre-vaccination to 71.4% post-vaccination.Conclusion: Varicella-zoster virus-specific T-cell-mediated immunity in pediatric hematopoietic stem cell transplant recipients remained low in the first 3 months but recovered significantly by 2 years post-hematopoietic stem cell transplant, with a boosting effect from varicella vaccination.Further studies are needed to assess the clinical significance of T-cell-mediated immunity and to guide personalized prevention strategies.