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Background: Varicella‐zoster virus (VZV) reactivation is the most common infectious complication in the late posthematopoietic stem cell transplantation (HSCT) period and is reported as 16%–41%. Acyclovir prophylaxis is recommended for at least 1 year after HSCT to prevent VZV infections. However, studies on the most appropriate prophylaxis are ongoing in pediatric patients. Methods: Patients who underwent allogeneic HSCT between January 1, 1996 and January 1, 2020 were retrospectively analyzed to outline the characteristics of VZV reactivation after allogeneic HSCT in pediatric patients using 6 months acyclovir prophylaxis. Results: There were 260 patients and 273 HSCTs. Median age was 10.43 (0.47–18.38), and 56% was male. Median follow‐up was 2325 days (18–7579 days). VZV reactivation occurred in 21.2% (n = 58) at a median of 354 (55–3433) days post‐HSCT. The peak incidence was 6–12 months post‐HSCT (43.1%). Older age at HSCT, female gender, history of varicella infection, lack of varicella vaccination, low lymphocyte, CD4 count, and CD4/CD8 ratio at 9 and 12 months post‐HSCT was found as a significant risk for herpes zoster (HZ) in univariate analysis, whereas history of varicella infection and low CD4/CD8 ratio at 12 months post‐HSCT was an independent risk factor in multivariate analysis. Conclusions: Tailoring acyclovir prophylaxis according to pre‐HCT varicella history, posttransplant CD4 T lymphocyte counts and functions, and ongoing immunosuppression may help to reduce HZ‐related morbidity and mortality. [ABSTRACT FROM AUTHOR]