Background and Objectives:Solid organ transplant (SOT) recipients have a higher risk of suffering from severe Coronavirus (COVID-19) compared to the general population. Studies have shown impaired immunogenicity of mRNA vaccines in this high-risk population; thus, SOT recipients have been prioritized globally for primary and booster doses.Materials and Methods:We analyzed 144 SOT recipients who had previously received two doses of BNT162b2 or mRNA1273 vaccine, and who were subsequently vaccinated with a booster dose of the mRNA1273 vaccine. Humoral and cellular immune responses were measured 1 and 3 months after the second dose, and 1 month after the third dose.Results:One month after the second dose, 33.6% (45/134) of patients displayed a positive antibody response with a median (25th, 75th) antibody titer of 9 (7, 161) AU/mL. Three months after the second dose, 41.8% (56/134) tested positive with a median (25th, 75th) antibody titer of 18 (7, 251) AU/mL. After the booster dose, the seropositivity rate increased to 69.4% (93/134), with a median (25th, 75th) titer of 966 (10, 8027) AU/mL. The specific SARS-CoV-2 T-cell response was assessed in 44 randomly selected recipients 3 months after the second dose, and 11.4% (5/44) of them had a positive response. Following the third dose, 42% (21/50) tested positive. Side effects after the third dose were mild, with pain at the injection site being the most frequent adverse effect, reported by 73.4% of the recipients.Conclusion:Our study shows a mild delayed increase in antibody titer, three months after primary vaccination compared to one month after. It also shows a robust augmentation of humoral and specific T-cell responses after the booster dose, as well as the safety and tolerability of the mRNA vaccines in SOT recipients.