A Study of mRNA-2808 in Participants With Relapsed or Refractory Multiple Myeloma

Key Inclusion Criteria: * RRMM with prior exposure to a proteasome inhibitor, an immunomodulatory drug (IMiD), and an anti-cluster of differentiation (CD38) monoclonal antibody. * Measurable disease defined as at least 1 of the following: * Serum M-protein ≥0.5 grams/deciliter * Urine M-protein ≥200 milligrams (mg)/24-hour * Involved free light chain (FLC) ≥100 mg/liter and an abnormal FLC ratio * Plasmacytoma with a single diameter ≥2 centimeters * Bone marrow plasma cells \>30% Key Exclusion Criteria: * Known central nervous system (CNS) myeloma or clinical signs and symptoms of CNS involvement of myeloma. * Active plasma cell leukemia, defined as peripheral blood plasma cells ≥20%. * Radiotherapy or cytotoxic chemotherapy within 2 weeks prior to Day 1 (Baseline), except palliative radiotherapy of limited field is permissible within 2 weeks after discussion with the Sponsor medical monitor. * Antibody-based immunotherapy (monoclonal antibody, bispecific antibody, antibody drug conjugate) within 21 days prior to Day 1 (Baseline). * Proteasome inhibitor therapy or immunomodulatory agent within 14 days prior to Day 1 (Baseline). * Autologous hematopoietic cell transplant within 100 days prior to Day 1 (Baseline). * Allogeneic hematopoietic cell transplant within 180 days prior to Day 1 (Baseline). * Genetically modified adoptive autologous or allogeneic cellular therapy (for example, chimeric antigen receptor T cell, chimeric antigen receptor natural killer) within 12 weeks prior to Day 1 (Baseline). * Corticosteroid therapy ≥140 mg prednisone or equivalent cumulative dose within 14 days prior to Day 1 (Baseline). Note: Other inclusion and exclusion criteria may apply.