Inclusion Criteria:  Individuals who meet all the following criteria are eligible for enrollment as study participants-  Able to understand and provide informed consent Recipient of a kidney or liver transplant ≥12 months prior to enrollment, without allograft rejection in the 6 months preceding enrollment Negative for anti-donor human leukocyte antigens (HLA) antibodies at screening (Central Lab Test Determination).  Currently taking one of the following calcineurin inhibitors (CNI)-based immunosuppressive regimens:  Tacrolimus plus Mycophenolate Mofetil (MMF) or Mycophenolic Acid (MPA), with or without a corticosteroid Tacrolimus with trough ≥ 5ng/mL with or without ≤5 mg of prednisone or equivalent Received a minimum of 2 doses of either the Moderna coronavirus infectious disease 19 (COVID-19) vaccine or Pfizer-BioNTech COVID-19 vaccine at least 30 days prior to study entry Serum antibody negative or indeterminate (titer <0.8 U/mL) at ≥ 30 days from the last dose of mRNA COVID-19 vaccine, measured using the Roche Elecsys®) severe acute respiratory syndrome coronavirus type 2 serological (anti-SARS-CoV-2) S assay, and Participant's transplant physician must confirm the participant's eligibility based on medical history and concur with the plan for immunosuppression modification.  Exclusion Criteria:  Individuals who meet any of these criteria are not eligible for enrollment as study participants-  Currently on an immunosuppressive regimen different from the three regimens described in the Inclusion Criteria, for example (but not limited to) those including sirolimus, everolimus, belatacept, or azathioprine Recipient of any allograft other than a kidney or liver Participant is pregnant Any past history of Donor Specific Antibody (DSA) using local site standards Currently taking any systemic immunosuppressive agent, other than their prescribed transplant immunosuppression Recipients of any COVID-19 vaccine other than the Moderna COVID-19 vaccine or the Pfizer-BioNTech COVID-19 vaccine Known history of severe allergic reaction to any component of an authorized or licensed COVID-19 vaccine Thrombotic events, myocarditis, or pericarditis temporally associated with a prior dose of COVID-19 vaccine History of heparin-induced thrombocytopenia Any change in transplant immunosuppression regimen (drug or dose) in response to suspected or proven rejection within the last 6 months More than minimal graft dysfunction, in accordance with study definition Receipt of any cellular depleting agent (e.g. antithymocyte globulins (ATG), rituximab, alemtuzumab, Cyclophosphamide) within 12 months preceding enrollment Concurrent autoimmune disease at risk for exacerbation with immunosuppression reduction Any untreated active infection including BK viremia >10^4 copies Infection with human immunodeficiency virus (HIV)  Recent (within one year) or ongoing treatment for malignancy with the exception of:  Non- melanomatous skin cancer definitively treated by local therapy, and Definitively treated carcinoma-in-situ of the cervix (Stage 0 cervical cancer) Treatment or prophylaxis of COVID-19 with a monoclonal antibody product or convalescent plasma within 6 months preceding enrollment, or  Any past or current medical problems, treatments, or findings which, in the opinion of the investigator, may:  pose additional risks from participation in the study, interfere with the candidate's ability to comply with study requirements, or impact the quality or interpretation of the data obtained from the study.