inclusion criteria: 1. age 18 years or older 2. received 2 doses of mrna-1273 (stratum b) according to the recommended vaccination schedule, with the last administration within the last nine months. for stratum a, also patients who received 2 doses of bnt162b2 and/or a third dose with a mrna vaccine (mrna-1273 or bnt162b2) within the last three months are eligible. 3. at least 6 months after kidney transplantation 4. negative seroresponse 14 to 56 days after vaccination, measured by a validated anti-spike igg assay of which the definition is dependent on the assay that is used. 5. eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) 6. capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) 7. willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: 8. maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone 9. in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l 10. in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l 11. prednisone dose at least 5 mg/day 12. first or second transplantation 13. calculated level of panel reactive antibodies prior to last transplantation below 85% 14. no signs of acute rejection during the preceding year

inclusion criteria: 1. age 18 years or older 2. received 2 doses of mrna-1273 (stratum b) according to the recommended vaccination schedule, with the last administration within the last nine months. for stratum a, also patients who received 2 doses of bnt162b2 and/or a third dose with a mrna vaccine (mrna-1273 or bnt162b2) within the last three months are eligible. 3. at least 6 months after kidney transplantation 4. negative seroresponse 14 to 56 days after vaccination, measured by a validated anti-spike igg assay of which the definition is dependent on the assay that is used. 5. eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) 6. capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) 7. willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: 8. maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone 9. in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l 10. in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l 11. prednisone dose at least 5 mg/day 12. first or second transplantation 13. calculated level of panel reactive antibodies prior to last transplantation below 85% 14. no signs of acute rejection during the preceding year

inclusion criteria: age 18 years or older received 2 doses of mrna-1273 (stratum b) according to the recommended vaccination schedule, with the last administration within the last nine months. for stratum a, also patients who received 2 doses of bnt162b2 and/or a third dose with a mrna vaccine (mrna-1273 or bnt162b2) within the last three months are eligible. at least 6 months after kidney transplantation negative seroresponse 14 to 56 days after vaccination, measured by a validated anti-spike igg assay of which the definition is dependent on the assay that is used. eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l prednisone dose at least 5 mg/day first or second transplantation calculated level of panel reactive antibodies prior to last transplantation below 85% no signs of acute rejection during the preceding year

inclusion criteria: age 18 years or older received 2 doses of mrna-1273 (stratum b) according to the recommended vaccination schedule, with the last administration within the last nine months. for stratum a, also patients who received 2 doses of bnt162b2 and/or a third dose with a mrna vaccine (mrna-1273 or bnt162b2) within the last three months are eligible. at least 6 months after kidney transplantation negative seroresponse 14 to 56 days after vaccination, measured by a validated anti-spike igg assay of which the definition is dependent on the assay that is used. eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l prednisone dose at least 5 mg/day first or second transplantation calculated level of panel reactive antibodies prior to last transplantation below 85% no signs of acute rejection during the preceding year

inclusion criteria: age 18 years or older received 2 doses of mrna-1273 according to the recommended vaccination schedule, with the last administration within the last nine months insufficient response to vaccination, defined as anti-spike igg in serum < 10 bau/ml measured between 25 and 56 days after the second dose of the mrna-1273 vaccine with a validated test eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l prednisone dose at least 5 mg/day first or second transplantation calculated level of panel reactive antibodies prior to last transplantation below 85% no signs of acute rejection during the preceding year

inclusion criteria: age 18 years or older received 2 doses of mrna-1273 according to the recommended vaccination schedule, with the last administration within the last nine months insufficient response to vaccination, defined as anti-spike igg in serum < 10 bau/ml measured between 25 and 56 days after the second dose of the mrna-1273 vaccine with a validated test eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l prednisone dose at least 5 mg/day first or second transplantation calculated level of panel reactive antibodies prior to last transplantation below 85% no signs of acute rejection during the preceding year

inclusion criteria: 1. age 18 years or older 2. received 2 doses of mrna-1273 according to the recommended vaccination schedule, with the last administration within the last nine months 3. insufficient response to vaccination, defined as anti-spike igg in serum < 10 bau/ml measured between 25 and 56 days after the second dose of the mrna-1273 vaccine with a validated test 4. eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) 5. capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) 6. willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: 7. maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone 8. in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l 9. in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l 10. prednisone dose at least 5 mg/day 11. first or second transplantation 12. calculated level of panel reactive antibodies prior to last transplantation below 85% 13. no signs of acute rejection during the preceding year

inclusion criteria: 1. age 18 years or older 2. received 2 doses of mrna-1273 according to the recommended vaccination schedule, with the last administration within the last nine months 3. insufficient response to vaccination, defined as anti-spike igg in serum < 10 bau/ml measured between 25 and 56 days after the second dose of the mrna-1273 vaccine with a validated test 4. eligible for covid-19 vaccination as described by the instructions of the manufacturers of the vaccine (moderna and janssen) 5. capable of understanding the purpose and risks of the study, fully informed and given written informed consent (signed informed consent form has been obtained) 6. willing to adhere to the protocol and be available during the study period additional inclusion criteria to be eligible for stratum a: 7. maintenance immunosuppressive therapy consisting of a calcineurin inhibitor (tacrolimus or cyclosporine), mmf/mpa, and prednisone 8. in case of tacrolimus treatment: last tacrolimus pre-dose level while on current dosage above 4 μg/l 9. in case of cyclosporine treatment: last cyclosporine pre-dose level while on current dosage above 75 μg/l 10. prednisone dose at least 5 mg/day 11. first or second transplantation 12. calculated level of panel reactive antibodies prior to last transplantation below 85% 13. no signs of acute rejection during the preceding year