Re‐vaccination against <scp>SARS</scp>‐<scp>CoV‐2</scp> in allogeneic <scp>HSCT</scp> patients: Repeated primary vaccine doses increase seroconversion rates

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.