Boyarsky BJ, Werbel WA, Avery RK, et al. - Researchers sought to assess antibody response among transplant recipients without prior polymerase chain reaction–confirmed COVID-19 after completion of the 2-dose SARS-CoV-2 mRNA vaccine series. A total of 658 transplant recipients who received 2 doses of SARS-CoV-2 mRNA vaccine were studied in this work. Detectable antibody responses were identified in majority after the second dose, although antibody levels were generally low among participants without a response after dose 1. Use of antimetabolite immunosuppression was noted to be persistently linked with poor humoral response. Although there is no established threshold for protective immunity, antibody levels were well below that which has been identified in immunocompetent vaccinees. Data herein suggest that a substantial proportion of transplant recipients likely remain at risk for COVID-19 after 2 doses of mRNA vaccine despite exhibiting an improvement in antispike antibody responses in transplant recipients after dose 2 compared with dose 1.