Booster Dose of COVID-19 Vaccine for Kidney Transplant Recipients Without Adequate Humoral Response

All recipients more than 6 months post transplantation and at least 3 weeks following second vaccine dose will be approached and invited to a first study visit. At first visit: * Signed informed consent will be obtained from participants willing to participate by study investigators who usually work in the transplantation clinic. * Anti-spike antibody response will be assessed using SARS-CoV-2 IgG II Quant (Abbott©) assay. Participants who have a documented seronegative test in the last 6 weeks will not be tested again. Participants will be invited for an additional visit once negative serology will be reported, within 7 days of serology collection. At this second visit all participants who gave informed consent to participate in either the prospective non-randomised study or RCT will receive a single vaccine dose. In addition, participants in the RCT will be randomised into two groups: 1. Third booster dose of BNT162b2 (one standard dose) with no change in immunosuppression protocol 2. Third booster dose of BNT162b2 (one standard dose) with immunosuppression reduction according to protocol (mycophenolic temporary cessation 4 days before (5 half-lives) and one week (expected antibody response) after vaccination (to allow for antibody response). Patients who will test seronegative will be informed by the study coordinator by phone in which study arm they will be participating and receive instructions for immunosuppression reduction both during the phone call and by written instructions provided to each patient during the first visit (see Appendix). Participants in the observational study will receive a third vaccine standard dose, without any change in immunosuppression (beyond routine care) For all groups: * Antibodies titer against spike protein will be evaluated again 2 weeks and 3, 6, 12 months after the third vaccine dose * T-cell response will be evaluated for a subset of patients in each group (estimated 20 patients per arm) before booster dose, at 2 weeks after booster dose, and at 3 months. For T cell response quantification peripheral blood mononuclear cell (PBMC) will be stimulated for 24 hours with spike protein and secreted interferon-gamma (IFNg) will be measured by ELISA. * Follow-up for adverse events, rejection and SARS-CoV-2 infection will be performed at 2 weeks and at three, 6 and 12 months post third vaccination dose