Safety and efficacy of the 3rd and the 4th dose of mRNA COVID19 vaccine in kidney transplant recipients

The study will enroll 140 kidney transplant recipients eligible for the study. All participants will have previously (winter and spring 2021) received a complete standard 2-dose vaccination schedule from Pfizer/BioNTech (or Moderna). Once enrolled in the study, participants will undergo a two-step randomization process, each step forming two subgroups in 1:1 ratio, with the use of random block sizes of 4. The first step of randomization determines whether the patient receives 1 or 2 booster doses (i.e., a total of 3 or 4 doses of vaccine 4 weeks apart of either BNT162b2 and mRNA-1273 vaccines), and the second step will determine which mRNA vaccine (either mRNA-1273, Moderna or BNT162b2, Pfizer/BioNTech) will be used (for 1 or both booster doses, respectively). The transplant nephrologist will be responsible for the randomization process. Sealed, sequentially numbered envelopes will be used to conceal the allocation to groups. The study will not be further masked. Immune response to vaccination will be measured at the baseline and 1 month after booster dose administration. Humoral response will be evaluated by measurement of SARS-CoV-2 specific IgG antibody titer. COVID19-specific cellular response will be investigated using an IFN-? ELISPOT performed on cryopreserved peripheral blood mononuclear cells using SARS-CoV-2 peptide pool as a stimulator. Alongside, relevant clinical data - especially adverse events attributable to vaccination, clinical efficacy of booster vaccination (occurrence of SARS-CoV-2 infection and its severity) – will be collected prospectively. A comparison of baseline and post-booster-dose COVID19-specific cellular and humoral response will be performed.