This is a multicenter prospective observational study that included a large cohort ( n = 397) of allogeneic (allo‐HSCT; ( n = 311) and autologous (ASCT) hematopoietic stem cell transplant ( n = 86) recipients who were monitored for antibody detection within 3–6 weeks after complete severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) vaccination from February 1, 2021, to July 20, 2021. Most patients ( n = 387, 97.4%) received mRNA‐based vaccines. Most of the recipients (93%) were vaccinated more than 1 year after transplant. Detectable SARS‐CoV‐2‐reactive antibodies were observed in 242 (78%) of allo‐HSCT and in 73 (85%) of ASCT recipients. Multivariate analysis in allo‐HSCT recipients identified lymphopenia < 1 × 10 9 /ml (odds ratio [OR] 0.33, 95% confidence interval [95% CI] 0.16–0.69, p = .003), active graft versus host disease (GvHD; OR 0.51, 95% CI 0.27–0.98, p = .04) and vaccination within the first year of transplant (OR 0.3, 95% CI 0.15–0.9, p = .04) associated with lower antibody detection whereas. In ASCT, non‐Hodgkin's lymphoma (NHL; OR 0.09, 95% CI 0.02–0.44, p = .003) and active corticosteroid therapy (OR 0.2, 95% CI 0.02–0.87, p = .03) were associated with lower detection rate. We report an encouraging rate of SARS‐CoV‐2‐reactive antibodies detection in these severe immunocompromised patients. Lymphopenia, GvHD, the timing of vaccine, and NHL and corticosteroids therapy should be considered in allo‐HSCT and ASCT, respectively, to identify candidates for SARS‐CoV‐2 antibodies monitoring.

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Dear Sir, ABO incompatibility is not a barrier for haematopoietic stem cell transplantation. In fact, ABO incompatibility between donor and recipient occurs in 30-40% of individuals and in more than 50% of those receiving umbilical cord blood transplantation (UCBT) (Konuma et al., 2014;Worel, 2016). A minor ABO mismatch occurs in 15-20% of human leucocyte antigen (HLA)-matched peripheral blood stem cell (PBSCT) (Confer et al., 2010). Minor ABO mismatch is defined when the donor's plasma is incompatible with the recipient's erythrocytes. This condition can produce immune complications such as immediate acute haemolysis and passenger lymphocyte syndrome (PLS) occurring during the first weeks after transplantation (Petz, 2005). A transition from isohaemagglutinin (IH) production by recipient B cells to maturing donor B cells occurs in patients undergoing peripheral blood and bone marrow transplantation (Lee et al., 2003). However, previous studies have shown a lack of IH production following minor ABO incompatible HLA mismatched UCBT (Snell et al., 2006;Igarashi et al., 2012), and consequently the question of whether immature B cells in UCBT ever produce any IH has been raised. The fact that cord blood contains a lower numbers of B cells and predominantly naïve B cells may lead to differences in IH production among patients undergoing cord blood and peripheral blood or bone marrow stem cell transplantation (Theilgaard-Monch et al., 2001). We retrospectively reviewed patients who underwent a minor ABO incompatible haematopoietic stem cell transplantation at our Institution at La Fe University Hospital, Valencia, and determined the presence of IH and titres in the patients who attended the hospital as outpatients. ABO, Rh typing and the indirect antiglobulin test (IAT) were determined using the automated system ORTHO Autovue Innova (Ortho Clinical Diagnostics, Marlow, England). The presence of IH was identified by reverse grouping and gel titrations in DG Gel Neutral Cards. Only IgM isohaemagglutinins were determined. Our objective was to assess and compare the long-term IH production pattern in patients who underwent an UCBT or PBSCT. A total of 32 patients were reviewed, 16 underwent UCBT and 16 PBSCT. Transplantation procedures, including conditioning regimes, graft vs host disease (GVDH) prophylaxis and supportive care, have been published previously by our group (Sanz et al., 2014). In all patients the ABO haematic group changed to the ABO

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