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)

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)

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)

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)