Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2) Immune Kidney Transplant Study (COVID-19)

Inclusion Criteria: * Patients transplanted with a kidney (only) at least 6 months before vaccination OR healthy volunteer first line health care workers at OUS. * Age of 18 years or older (also for controls). * Standard immunological risk at transplantation (i.e. no donor specific HLA antibodies (DSA)) and not performed an ABO blood-type-incompatible transplantation. * No treatment for rejection episodes the last 6 months before inclusion. * Immunosuppressive therapy including tacrolimus, mycophenolate and prednisolone. * Stable graft-function the last 6 months. * S-creatinine \< 200 μmol/L (also for controls). * Signed informed consent to participate in the study (also for controls). Exclusion Criteria: * Three or more previous transplantations. * Hemoglobin level below 10 g/dL (also for control). * Leukopenia defined as total lymphocyte count \< 2 X 109 (also for controls). * Previous treatment with anti-thymocyte antibodies (ATG) or rituximab.