RECOVAC Repeated Vaccination Study

2\. OBJECTIVES Primary objective: To assess the immunogenicity (expressed as percentage of responders) of various COVID-19 third or fourth vaccination strategies in kidney transplant patients that failed to mount a sufficient antibody response after two or three primary doses of a mRNA vaccine (either mRNA-1273 (Moderna) or BNT162b2 (Pfizer) or any combination of both). Secondary Objectives: * To measure the concentration of SARS-CoV-2 spike S1-specific IgG antibodies in serum at 28 days after the 3rd or 4th vaccine administration * To measure the presence and titer of neutralizing anti-SARS-CoV-2 antibodies after third or fourth vaccination * To evaluate SARS-CoV-2 specific T cell responses * To measure anti-S1 antibody (IgG and IgA) responses and neutralizing capacity of these antibodies in nasal mucosal fluid * To evaluate vaccine safety in terms of incidence of solicited local and systemic adverse events (AEs) graded according to severity. Exploratory Objectives: * To assess the association between baseline clinical and immunological parameters and the immune response to third or fourth vaccinations * To correlate RNA-seq data at one week after vaccination to the subsequent antibody and T cell response 3\. STUDY DESIGN This is a prospective, multicentre, open-label, randomized study to evaluate the immunogenicity and safety of various third or fourth vaccination strategies in kidney transplant patients without a sufficient antibody response after two or three primary doses of a mRNA vaccine (either mRNA-1273 (Moderna) or BNT162b2 (Pfizer) or any combination of both). The four participating study sites are: Amsterdam UMC, ErasmusMC, Radboudumc and UMCG. Based on their immunosuppressive treatment, patients will participate in one of the following strata: * stratum A: patients treated with triple immunosuppressive therapy consisting of a calcineurin inhibitor, MMF/MPA, and steroids, * stratum B: patients treated with any combination of immunosuppressive drugs. In stratum A, patients will be randomized to one of two third or fourth vaccination strategies: * A1: one extra dose of mRNA-1273 (100 μg, i.m) * A2: one extra dose of mRNA-1273 (100 μg, i.m), with temporary discontinuation of MMF/MPA during one week before and one week after the 3rd dose In stratum B, patients will be randomized to one of three third vaccination strategies: * B1: 3rd dose of mRNA-1273 (100 μg, i.m) * B2: 3rd dose of mRNA-1273 (100 μg, i.m) in both upper arms * B3: Ad26.COV2.S vaccine (Janssen, 5x1010 viral particles i.m.) To assess the immune response after vaccination, blood samples will be collected at baseline (i.e. prior to third dose administration) and at 28 days after the third or fourth dose administration. A complete blood count, kidney function, and liver enzymes will be determined at the same time points. In a subset of patients we will collect a blood sample at 1 week after the third vaccin administration for specific immunological assays. For safety reasons, a blood sample will be collected in stratum A at one and two weeks after discontinuing MMF/MPA. The maximum volume of blood collected for this study is 66 mL per timepoint, and no more than 200 mL for the one year study period, which is well within the safe limits of blood collection for clinical studies. Nasal mucosal lining fluid (MLF) will be collected for assessment of secreted mucosal antibodies at the same time points used to collect blood. MLF will be collected by use of soft synthetic absorptive matrix (SAM) strips, which are gently inserted into one nostril of the participant and placed along the surface of the inferior turbinate. The index finger is then lightly pressed against the side of the nostril to keep the SAM strip in place and to allow MLF absorption for 60 seconds, after which the SAM strip is placed back in the protective plastic tube. The procedure may tickle slightly but is completely painless. To evaluate vaccination related AEs, patients will be asked to collect solicited local and systemic AEs for 7 days after each vaccination using a questionnaire, as vaccination related AEs are mainly expected in the first week after vaccination.