The
COVID-19 pandemic threatens patients with a compromised immune and endothelial system, including patients who underwent allogeneic
stem cell transplantation (alloSCT). Thus, there is an unmet need for optimizing vaccination management in this high-risk cohort. Here, we monitored
antibodies against
SARS-CoV-2 spike protein (anti-S1) in 167 vaccinated alloSCT patients. Humoral immune responses were detectable in 81% of patients after two vaccinations with either
mRNA-, vector-based, or heterologous regimens. Age, B-cell counts, time interval from vaccination, and the type of
vaccine determined antibody titres in patients without systemic immunosuppression (sIS). Similar to a healthy control cohort,
mRNA vaccine-based regimens induced higher titres than vector-based
vaccines. Patients on two or more
immunosuppressants rarely developed immunity. In contrast, 62% and 45% of patients without or on only one
immunosuppressant, respectively, showed a strong humoral vaccination response (titre > 100). Exacerbation of cGVHD upon vaccination was observed in 6% of all patients and in 22% of patients receiving immunosuppression for cGVHD. cGVHD exacerbation and low antibody titres were both associated with higher
angiopoietin-2 (ANG2) serum levels. In conclusion,
mRNA-based
vaccines elicit strong humoral responses in alloSCT patients in the absence of double sIS.
Biomarkers such as ANG2 might help with weighing cGVHD risk versus beneficial responses.