Allogeneic hematopoietic stem cell transplantation (allo-HSCT) recipients were excluded from the initial SARS-CoV-2 mRNA vaccination efficacy trials. Suboptimal vaccine responses have been reported in immunocompromised cohorts such as patients with solid tumors or hematologic malignancies, suggesting the need for additional research. Widespread data on the antibody responses and vaccine efficacy in allo-HSCT recipients is limited. In our single-center, retrospective study, we analyzed the anti-spike IgG antibody responses in 75 allo-HSCT recipients who received a series of two doses of mRNA vaccination. We collected data on previous COVID-19 infection, B and T lymphocyte recovery, donor types, graft-vs.-host disease (GVHD), and immunosuppressive medications at the time of vaccination. With the original variant, a cutoff of 4,160 arbitrary units (AU)/mL has been correlated with a 0.95 probability of a viral neutralization. We also examined the number of allo-HSCT recipients who achieved this conservative threshold. To our knowledge, no correlate exists for the currently prevalent Omicron variant and viral neutralization. Despite 29.3% (22/75) of patients being on systemic immunosuppressive medications due to chronic GVHD, positive antibody responses > 50 AU/mL were seen in 96% of patients. However, only 48% (36/75) of patients were above the neutralizing antibody threshold. Those with previous COVID-19 infection had significantly higher antibody responses. Although encouraging, the variability of the responses underscores the concept of ongoing antibody monitoring as well as consideration of additional doses of the COVID-19 vaccine in this cohort.
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