Summary The immunogenicity and safety of Pfizer‐BioNTech BNT162b2 mRNA vaccine in allogeneic haematopoietic stem cell transplantation (HSCT) recipients are unknown. We prospectively followed 152 HSCT recipients who were at least six months following transplantation and with no active acute graft‐ versus ‐host disease (GVHD). Blood samples were taken 2–4 weeks after the second vaccination and analyzed for receptor‐binding domain (RBD) antibodies and neutralizing antibodies (NA). 272 immunocompetent healthcare workers served as controls. At a median of 28 days after the second vaccination, 118 patients (77·6%) developed RBD immunoglobulin G (IgG) with a geometric mean titre (GMT) of 2·61 [95% CI (confidence interval), 2·16–3·16]. In the control group 269/272 (98·9%) developed RBD IgG, with a GMT of 5·98 (95% CI 5·70–6·28), P < 0·0001. The GMT of NA in HSCT recipients and controls was 116·0 (95% CI 76·5–175·9), and 427·9 (95% CI 354·3–516·7) respectively ( P < 0001). Multivariate logistic regression analysis revealed that HSCT recipients with no chronic GVHD and no immunosuppressive therapy at the time of vaccination had significantly higher levels of NA following the second vaccination. Adverse events were minimal and were less common than in healthy controls. In conclusion; the BNT162b2 mRNA vaccination is safe and effective in HSCT recipients, especially those who are immunosuppression‐free. A significant fraction developed protecting NA.
Allogeneic hematopoietic SCT is a curative treatment for a variety of hematological malignancies and genetic diseases. There is a continuous search for novel conditioning regimens that will reduce SCT-related toxicity while retaining maximal antimalignancy effect. Treosulfan (L-threitol-1,4-bis-methanesulfonate; dihydroxybusulfan) was initially used in the treatment of certain solid tumors. Preclinical studies showed that it has a myeloablative effect on committed and non-committed stem cells. It has potent immunosuppressive characteristics, more prominent than its related chemotherapy agent BU, which makes it an attractive candidate for the use in conditioning regimens before allo-SCT. It is also associated with a favorable toxicity profile with little extramedullary toxicity. The combination of fludarabine and treosulfan was explored in several studies in patients not eligible for standard myeloablative conditioning, and data are rapidly emerging. This regimen is associated with consistent engraftment. A limited non-relapse mortality (NRM) rate in the range of 9-28% was observed. This rate is promising considering the patients selected and results from low rates of organ toxicity as well as acute and chronic GVHD. The regimen was also associated with low relapse rates of 5-30% depending on disease status at SCT. Together with low NRM rate, this resulted in favorable survival in the range of 40-80%. Promising results were seen in myelodysplastic syndrome (survival 36-70%) and leukemia in remission (60-70%). Randomized prospective studies will be needed to better define the role of treosulfan-based regimens in SCT.
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