Inclusion Criteria:  kidney transplant recipients that received two doses of BNT162b2 vaccine at least 3 weeks prior to enrollment, and were seronegative (IgG against the spike protein of SARS-CoV-2 below 50 AU/ml) at least two weeks after the second vaccine dose  Additional inclusion criteria for the RCT:  Recipients treated by three anti-rejection medications including: prednisone, tacrolimus, mycophenolate mofetil or mycophenolic acid. Tacrolimus trough blood levels 5-10 nGr/ml (lower or higher doses will have to be adjusted before re-considering for inclusion)  Exclusion Criteria:  Past infection with SARS-CoV-2 Pregnancy Age below 18 years Active infection  Additional exclusion criteria for RCT only:  - Recipients at a high risk for acute or chronic humoral rejection including:  Recipients with positive panel-reactive antibody (PRA) (any positive value) at any time before or after transplantation Recipients that had an acute rejection in the last year Recipients less than 6 months after transplantation Recipients that are considered at high risk for rejection according to the primary care nephrologist Recipients taking less than 3 anti-rejection medications Recipients currently treated with mTOR inhibitors (everolimus, sirolimus) and/or azathioprine Recipients treated with plasmapheresis in the previous 3 months Recipients treated with eculizumab in the last year Recipient treated with IVIG in the previous 3 months Recipient treated with rituximab in the previous 6 months