Patients (pts) who have undergone allogeneic stem cell transplantation (alloSCT) are at high-risk for life-threating complications post SARS-CoV-2 infection, and the mortality rates has been reported of approximately 30-35%. The currently available vaccines proved their effectiveness in the general population by reducing the severity of the COVID-19 infection however, scant data exist regarding the safety and efficacy of the commercially available vaccines in allografted pts.

After a median of 2,7 (0,3-6,7) ys post alloSCT, 20 pts received within a median of 42 days, 2 vaccines of either Pfizer (n=17) or combinations of Pfizer with Moderna (n=2) or AstraZeneca (n=1). Off immunosuppression without evidence of active GvHD were 14 pts, 1 was only on Cyclosporine (CSP) while 5 were on steroids plus CSP or MMF or Ibrutinib for GvHD treatment. Automated commercial chemiluminescence immunoassay (CLIA) against spike (S1/S2) protein was used for antibody responses detection.

During vaccination program no side effect grade ≥3 (including allergy, thrombosis, heart dysfunction or laboratory abnormalities) was reported. The commonest complains were fatigue (20%), bony pain (10%) and fever <38.5 oC (10%). Satisfactory antibody responses were observed in 66% and 95% of pts after the 1st and 2nd dose respectively. Importantly, active GvHD and intensive immunosuppression, did not negatively affect the antibody responses. None of the vaccinated pts developed COVID infection

Our retrospective study although with small number of patients and with short term follow-up, in agreement with others, confirms that the current commercially available vaccines against SARS-CoV-2 are safe and highly effective in producing effective humoral responses in allografted patients. Prospective studies with longer follow-up are needed to elucidate the proper timing and the number of necessary doses for a safe and effective approach in preventing severe COVID-19 infection