LAUSR

Allogeneic haematopoietic stem cell transplant (HSCT) recipients remain at high risk of adverse outcomes from coronavirus disease 2019 (COVID‐19) and emerging variants. The optimal prophylactic vaccine strategy for this cohort is not defined. T cell‐mediated immunity is a critical component of graft‐versus‐tumour effect and in determining vaccine immunogenicity. Using validated anti‐spike (S) immunoglobulin G (IgG) and S‐specific interferon‐gamma enzyme‐linked immunospot (IFNγ‐ELIspot) assays we analysed response to a two‐dose vaccination schedule (either BNT162b2 or ChAdOx1) in 33 HSCT recipients at ≤2 years from transplant, alongside vaccine‐matched healthy controls (HCs). After two vaccines, infection‐naïve HSCT recipients had a significantly lower rate of seroconversion compared to infection‐naïve HCs (25/32 HSCT vs. 39/39 HCs no responders) and had lower S‐specific T‐cell responses. The HSCT recipients who received BNT162b2 had a higher rate of seroconversion compared to ChAdOx1 (89% vs. 74%) and significantly higher anti‐S IgG titres (p = 0.022). S‐specific T‐cell responses were seen after one vaccine in HCs and HSCT recipients. However, two vaccines enhanced S‐specific T‐cell responses in HCs but not in the majority of HSCT recipients. These data demonstrate limited immunogenicity of two‐dose vaccination strategies in HSCT recipients, bolstering evidence of the need for additional boosters and/or alternative prophylactic measures in this group.