Re‐vaccination against SARS‐CoV‐2 in allogeneic HSCT patients: Repeated primary vaccine doses increase seroconversion rates.

Summary: Patients with haematological malignancies often exhibit reduced antibody responses to severe acute respiratory syndrome‐related coronavirus vaccines, especially those who have undergone allogeneic haematopoietic stem cell transplantation (HSCT). Limited data exist on vaccine efficacy in this group. In a retrospective analysis of 75 post‐HSCT patients, we assessed serologic responses to one to four doses of Pfizer‐BioNTech (PB), AstraZeneca (AZ) or Moderna (MU) vaccines within 2 years post‐transplant. Seroconversion rates were 50.7%, 78%, 79% and 83% after the first to fourth doses respectively. The median time from allograft to first re‐vaccination was 145 days (range 79–700). Failure to respond to the first vaccine dose was linked to acute GVHD (p = 0.042) and rituximab treatment within 12 months (p = 0.019). A trend was observed with chronic GVHD and seroconversion failure after the second (p = 0.07) and third (p = 0.09) doses. Patients vaccinated before HSCT showed better antibody responses post‐transplant (p = 0.019). Coronavirus disease 2019 incidence was 16%, with 17% hospitalized and one death (8%). Despite low initial seroconversion rates post‐HSCT, antibody responses improved after the second dose. Early full re‐vaccination and boosters post‐HSCT are recommended to reduce mortality. Rituximab use and active GVHD were identified as risk factors, warranting further investigation. [ABSTRACT FROM AUTHOR]