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  Exclusion Criteria:  Multi-organ transplant recipient History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (e.g. anaphylaxis) to any component of the study intervention(s). Previous or active COVID-19 disease Active malignancy, except non-melanoma skin cancer Inherited immune deficiency Infection with Human Immunodeficiency Virus (HIV) Administration of T cell, B cell, or plasma cell depleting antibodies during the last 6 months Any vaccination within a month before enrolment Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection.