COVID-19 vaccine protection against infection in immunosuppressed solid organ transplant recipients is unknown but possibly weak with the low proportion of these patients mounting a robust humoral and cellular immune response after vaccination. Using a retrospective cohort study design with cross-over, we assessed vaccine effectiveness among 782 kidney transplant recipients registered at Hamad Medical Corporation, the national public healthcare provider in Qatar, where the BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) vaccines have been used in the national immunization campaign. Vaccine effectiveness against any SARS-CoV-2 infection was estimated at 46.6% (95% CI: 0.0-73.7%) ≥14 days after the second dose, 66.0% (95% CI: 21.3-85.3%) ≥42 days after the second dose, and 73.9% (95% CI: 33.0-89.9%) ≥56 days after the second dose. Vaccine effectiveness against any severe, critical, or fatal COVID-19 disease was estimated at 72.3% (95% CI: 0.0-90.9%) ≥14 days after the second dose, 85.0% (95% CI: 35.7-96.5%) ≥42 days after the second dose, and 83.8% (95% CI: 31.3-96.2%) ≥56 days after the second dose. Most vaccine breakthrough infections occurred in the first few weeks after receiving the first and/or second dose. Vaccine effectiveness reached considerable levels in kidney transplant recipients, but vaccine protection mounted slowly and did not reach a high level until several weeks after the second dose.