This is a summary, written by members of the CITF Secretariat, of:

Kimura M, Ferreira VH, Kothari S, Pasic I, Mattsson JI, Kulasingam V, Humar A, Mah A, Delisle JS, Ierullo M, Majchrzak-Kita B, Kumar D, Hosseini-Moghaddam SM. Safety and Immunogenicity After a Three-Dose SARS-CoV-2 Vaccine Schedule in Allogeneic Stem Cell Transplant Recipients. Transplant Cell Therapy. 2022 Oct;28(10): 706.e1-706.e10. doi: 10.1016/j.jtct.2022.07.024

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study, published in Transplant Cell Therapy, from Dr. Deepali Kumar’s team (University of Toronto) shows that a third dose of COVID-19 vaccine resulted in stronger humoral Humoral immunity is mediated by antibodies that are produced by the B-cells. and cell-mediatedCell-mediated immunity is the response that does not rely on antibodies. It is mainly mediated by the T-cells, but also involves the activation of other white blood cells such as macrophages, and the production of soluble molecules called cytokines. immunity in allogeneic stem cell transplantA transplant of blood stem cells to a person whose bone marrow does not produce enough healthy blood cells. In allogeneic transplants, stem cells are taken from a donor and given to a recipient. (Allo-SCT) recipients, when compared to the initial two-dose vaccine regimen. The third dose did not increase the risk of graft-versus-host disease (GVHD), which results from the attack of the recipient’s body by the donor’s stem cells, or other adverse effects.

Key findings:

  • Humoral responses against the receptor-binding domain (RBD) of the spike protein were generated by a majority (89.1%) of Allo-SCT recipients who received 3 doses of COVID-19 vaccine.
  • The median level of anti-RBD antibodies was significantly higher after the third COVID-19 vaccine dose when compared to the first and second doses (10,358.0 U/mL versus 10.2 U/mL and 125.6 U/mL, respectively).
  • Having a haploidentical Haploidentical donors are usually family members whose tissue type is half-matched to that of the recipient. donor was found to be a risk factor for suboptimal humoral response (defined as an anti-RBD antibody titer below 100 U/mL).
  • Regarding cell-mediated immunity, most Allo-SCT recipients developed antigen-specific CD4+ (between 55% and 80% of participants, depending on cell subtype) and CD8+ T-cells (80% of participants) after the second dose of vaccine. These proportions remained as high or increased slightly, depending on the cell subtype, after the third dose.
  • 23 episodes of new onset or worsening of pre-existing GVHD occurred after receiving a dose of COVID-19 vaccine (representing 7.6% of all doses received by all patients). Of these, only 4 occurred after a third dose.

A total of 122 Allo-SCT recipients were included in the analysis, of whom 74 (60.7%) received 3 doses of SARS-CoV-2 vaccine. Adult patients who received a COVID-19 vaccine dose 3 months after undergoing Allo-SCT in Toronto​​​ were invited to participate in the study, beginning in August 2021. Those who received at least 1 dose of COVID-19 vaccine before December 2021 were included and followed up until February 2022.