Publication date: Jul 03, 2025

Background: Allogeneic hematopoietic stem cell transplant (Allo-HSCT) recipients are still at increased risk of severe COVID-19 infection. Vaccination is a critical strategy to protect this population. This real-world prospective cohort study aimed to evaluate the immune response and clinical outcomes of COVID-19 vaccines in Allo-HSCT recipients. Methods: Allo-HSCT recipients (median age: 48 years) who received either the BNT162b2 or CoronaVac vaccines were included. Antibodies against the SARS-CoV-2 spike protein were quantitatively measured using the chemiluminescent microparticle immunoassay. Patient- and vaccine-related factors affecting antibody responses were analyzed. Adverse events, including graft-versus-host disease (GVHD) and post-vaccine infections, were recorded. Results: Among 95 Allo-HSCT recipients, 86. 3% achieved adequate antibody responses following COVID-19 vaccination. Patients receiving ≥3 vaccine doses showed significantly higher antibody titers compared to those with only 2 doses (OR: 0. 11; 95% CI: 0. 02-0. 53; p = 0. 006 **). The use of Ruxolitinib or Ibrutinib was associate with increased odds of low antibody response (OR: 38. 39; 95% CI: 3. 14-468. 95; p = 0. 004 **). Hypogammaglobulinemia (low serum IgG levels) was associated with a reduced antibody response (OR: 0. 17; 95% CI: 0. 03-0. 96; p = 0. 045 *), while no significant correlation was found between serum IgA levels and antibody responses (p = 0. 672). Three cases of post-vaccine GVHD were observed, and no fatalities related to COVID-19 occurred during the study. Conclusions: COVID-19 vaccination is safe and effective in Allo-HSCT recipients, with stronger responses especially following ≥3 vaccine doses. Patients receiving GVHD treatment or with hypogammaglobulinemia exhibited impaired responses, emphasizing the need for tailored vaccination strategies and close monitoring in this population.

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