LAUSR

HCT recipients reportedly have a high mortality rate after developing COVID‐19. SARS‐CoV‐2 vaccination is generally useful to prevent COVID‐19. However, its safety and efficacy among HCT recipients remain elusive. This large‐scale prospective observational study including 543 HCT recipients with 37‐months interval from transplant demonstrated high safety profiles of mRNA vaccine: only 0.9% of patients avoided the second dose due to adverse event or GVHD aggravation following the first dose. Regarding the efficacy, serological response with a clinically relevant titer (≥250 BAU/mL) was obtained in 397 (73.1%) patients. We classified the remaining 146 patients as impaired responders and compared the clinical and immunological parameters between two groups. In allogeneic HCT recipients, multivariable analysis revealed the risk factors for impaired serological response as follows: age (≥60, 1 points), HLA‐mismatched donor (1 points), use of systemic steroids (1 points), absolute lymphocyte counts (<1000/μL, 1 points), absolute B‐cell counts (<100/μL, 1 points), and serum IgG level (<500 mg/dL, 2 points). Notably, the incidence of impaired serological response increased along with the risk scores: patients with 0, 1–3, and 4–7 points were 3.9%, 21.8%, and 74.6%, respectively. In autologous HCT recipients, a shorter interval from transplant to vaccination was the only risk factor for impaired serological response. Our findings indicate that two doses of SARS‐CoV‐2 vaccine are safe but insufficient for a part of HCT recipients with higher risk scores. To improve this situation, we should consider additional treatment options, including booster vaccination and prophylactic neutralizing antibodies during the SARS‐CoV‐2 pandemic.