BACKGROUND: Messenger RNA vaccination against COVID-19 has been shown to produce an immune response with sufficient efficacy to prevent natural infection in immunocompetent recipients. However, the response in kidney transplant recipients is low. We aimed to evaluate the specific humoral response to SARS-CoV-2 after vaccination in a population of kidney transplant recipients and assess the main factors associated with a lack of response. METHODS: We undertook a prospective study of 105 kidney transplant recipients and 11 recipients of a combined kidney-pancreas transplant. We analyzed immunoglobulin G and immunoglobulin M antibodies after the patients received their second and third doses of the messenger RNA 1273 (Moderna) or BNT162b1 (BionTECH-Pfizer) vaccinations between February and November 2021. RESULTS: Mean (SD) age of the 116 patients was 50 (16) years, and 65% were men. They had their transplants for 40 months (IQR, 15-123 months), with 14% undergoing retransplant and 11% sensitized. The maintenance immunosuppression regimen was steroids + tacrolimus + mycophenolate (MMF) in 68% of the patients and any combination with mammalian target of rapamycin inhibitor (mTORi) in 28%. A humoral response developed in 40% of the patients 6 weeks (IQR, 4-10 weeks) after receiving the second dose of the vaccine. Of the 67 patients with no response to the second dose, 51 had an analysis of the humoral response after the third dose, which was positive in 16 (31%). A total of 80% received the Moderna vaccine and 20% the BionTECH-Pfizer. No patient experienced major adverse effects after the vaccination. Factors associated with a lack of humoral response to the vaccine were recipient age (odds ratio [OR], 1.02; 95% CI, 1.001-1.05; P = .04), diabetes (OR, 2.8; 95% CI, 1.2-6.9; P = .02), and treatment with MMF (OR, 2.6; 95% CI, 1.08-6.8; P = .03). Treatment with mTORi was associated with a better response to vaccination (OR, 0.3; 95% CI, 0.1-0.9; P = .04). CONCLUSIONS: The humoral response to the COVID-19 vaccine in kidney transplant recipients is poor. Factors related with this lack of immunity are recipient age and diabetes, plus MMF therapy, whereas mTORi therapy was associated with a better response to vaccination.