RECOVAC Repeated Vaccination Study

Rationale: The humoral and cellular immune response after two mRNA vaccinations is severely attenuated in kidney transplant patients compared to controls, especially when their immunosuppressive regimen contains mycophenolate mofetil (MMF) / mycophenolic acid (MPA). A repeated dose strategy is therefore required to improve the efficacy of vaccination. Objective: To investigate the immunogenicity of third or fourth dose SARS-CoV-2 vaccination strategies in kidney transplant patients. Study design: Prospective, multicentre, open-label randomized clinical trial Study population: Patients with a functioning kidney transplant who did not seroconvert after two or three doses of a mRNA vaccine (either mRNA-1273 (Moderna) or BNT162b2 (Pfizer) or any combination of both) Procedures: Based on their immunosuppressive treatment, patients can participate in one of the following strata: * stratum A: patients receiving triple immunosuppressive therapy, consisting of a calcineurin inhibitor, MMF/MPA, and steroids In stratum A, patients will be randomized to one of two equally sized groups. Patients will receive a third or fourth vaccination of the mRNA-1273 vaccine (100 μg, i.m), with either continuation of MMF/MPA (A1) or discontinuation of MMF/MPA during one week before and one week after the third or fourth dose, respectively (A2). * stratum B: patients receiving any combination of immunosuppressive drugs. In stratum B, patients will be randomized to one of three equally sized groups. Patients will receive another dose (100 μg, i.m) of the mRNA-1273 vaccine (B1), or two single doses of mRNA-1273 into the left and the right upper arm (2 x 100 μg, i.m; B2), or the Ad26.COV2.S vaccine (Janssen, 5x1010 viral particles, i.m; B3). Main study parameters/endpoints: The primary endpoint is the proportion of patients with an anti-S1 antibody concentration higher than 10 BAU/mL established at 28 days after the third or fourth vaccine administration. Within each stratum different va