120 Transplant recipientswithout previous history of COVID who had received two doses of mRNA-1273 were randomly assigned in a 1:1 ratio to receive either a third dose of mRNA-1273 vaccine or saline placebo 2 months after the second dose of mRNA-1273 (dosing schedule: 0, 1, and 3 months). At month 4, an anti-RBD antibody level of at least 100 U per milliliter was present in 33 of 60 patients (55%) in the mRNA-1273 group and in 10 of 57 patients (18%) in the placebo group (relative risk, 3.1; 95% confidence interval [CI], 1.7 to 5.8; P<0.001). After the third dose, the median percent virus neutralization was 71% in the mRNA-1273 group and 13% in the placebo group (95% CI for the between-group difference, 11 to 76 percentage points), and the percentage of patients above the 30% threshold for neutralizing antibody positivity was 60% and 25%, respectively (relative risk, 2.4; 95% CI, 1.5 to 4.0). Median severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–specific T-cell counts were greater after the third dose in the mRNA-1273 group than in the placebo group (432 vs. 67 cells per 106 CD4+ T cells; 95% CI for the between-group difference, 46 to 986). This study suggested that a third-dose booster Covid-19 vaccine should be considered, in conjunction with regulatory approval, for transplant recipients who have received two doses of mRNA-1273.