Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection results in poor outcome in patients with hematologic malignancies. Moreover, the efficacy of anti-SARS-CoV-2 mRNA vaccines appears lower in immunocompromised patients, including recipients of allogeneic stem cell transplantation (Allo-HSCT). In this population, data are scarce regarding factors predicting the response to mRNA vaccines.


This retrospective study aimed to decipher which factors, including immune status at time of vaccine and recipient/donor blood groups, might influence the antibody response after two injections (V1 and V2) of BNT162b2 (Pfizer-BioNTech) vaccine in a cohort of allografted patients with no previous symptomatic nor asymptomatic COVID-19 infection. Possible previous asymptomatic COVID-19 infection was investigated in pre-V1 samples by testing for anti-nucleocapsid (N) antibodies (anti-SARS-CoV-2 immunoassay, Roche Elecsys®, Rotkreuz, Switzerland). Antibody response to the SARS-CoV-2 spike protein receptor-binding domain was tested post-V2 (Roche Elecsys®). As recommended by the manufacturer, titers ≥0.8 U/mL were considered positive, the highest value being >250 U/mL.

Blood samples were also collected before V1 and at distance from V2 to evaluate, by flow cytometry, total lymphocyte (Ly) counts and quantitative Ly subsets (CD3, CD4 and CD8 T cells, B and NK cells).

Statistical analyses were performed on R (version 4.0.3). Patient characteristics were compared by using the Χ² test for discrete variables and the Wilcoxon test for continuous variables. Generalized linear models were used to conduct multivariate analyses.


Samples were available from 117 Allo-HSCT patients who had been vaccinated between January 20 and April 17, 2021. Patient characteristics are provided in Table 1. The average interval from Allo-HSCT day 0 (D0) to V1 (D0-V1) was 654 (IQR: 372-1367) days (d). S-antibody response rate post-V2 was 82.9% for the entire cohort. Non-humoral responders (NHR) post-V2 (n = 20) had a lower D0-V1 interval (median 271 vs 914 d, p <10 -5) and a lower pre-V1 median total Ly count (0.62 vs 1.61x10 9/L, p < 10 -4). Lymphocyte subsets possibly predictive of antibody response were then investigated. NHR were associated with lower median CD3 (0.39 vs 0.97 x10 9/L, p = 0.01), CD4 (0.13 vs 0.35 x10 9/L, p=<10 -3), and B-cell (0.00 vs 0.28 G/L, p <10 -6) counts. NK and T CD8 counts were not statistically different between NHR and HR (respectively p=0.14 and p=0.06). No influence either was observed when considering the age of donors (p=0.39) or recipients (p=0.55), underlying disease (p=1), Allo-HSCT conditioning (p=0.11), blood groups (donor, p=0.55; receiver, p=0.39) or a previous history of graft versus host disease (GVHD; 83.1 vs 83.6%, p=1). Conversely, ongoing immunosuppressive (IS)/chemotherapy treatment and a haploidentical source of graft were associated with lower responses to vaccination (respectively 62.5 vs 90.5%, p<10 -3, and 69.4 vs 88.6% for patients with matched donors, p=0.02). In multivariate analysis (Fig.1) also including D0-V1 interval, donor source, current IS/chemotherapy treatment and TCD4 Ly count, only B cell aplasia remained statistically associated with lack of antibody response after two vaccine injections (OR 0.01, 95%CI [0.00 - 0.10], p <10 -3).

The possible modification in terms of lymphocyte counts between pre-V1 and post-V2 times has been also investigated showing that only CD4 lymphocytes counts improved significantly (0.31 vs 0.34 x10 9/L, p=0.01) between this interval.


B cell aplasia appears as a major predictor of anti SARS-CoV-2 mRNA vaccine failure after Allo-HSCT. It may be suggested from this result that a close immune monitoring should be proposed after allotransplant to propose the vaccine at the most appropriate time, meaning after of B cell detection, regardless of the delay from Allo-SCT or the presence of an IS/chemotherapy treatment. The possibility for these patients to have mounted a cellular response should also be considered, which was not investigated here.


Moreau:Celgene BMS: Honoraria; Sanofi: Honoraria; Abbvie: Honoraria; Oncopeptides: Honoraria; Amgen: Honoraria; Janssen: Honoraria.

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