A Study to Compare the Efficacy and Safety of Idecabtagene Vicleucel With Lenalidomide Maintenance Therapy Versus Lenalidomide Maintenance Therapy Alone in Adult Participants With Newly Diagnosed Multiple Myeloma Who Have Suboptimal Response After Autologous Stem Cell Transplantation

Inclusion Criteria * Participants aged ≥18 with Newly Diagnosed Multiple Myeloma (NDMM) who has received induction therapy followed by high-dose chemotherapy and autologous stem cell transplantation (ASCT), without subsequent consolidation or maintenance. EXCEPTION: Participant received ≤ 7 days of lenalidomide (LEN) maintenance therapy and the investigator documents that there is no impact to the overall benefit/risk assessment due to the temporary interruption of LEN. * Participant must have received 4 to 6 cycles of induction therapy, which must contain at a minimum an immunomodulatory drugs (IMiD) and a proteasome inhibitor (PI) (with or without anti-CD38 monoclonal antibody) and must have had a single ASCT 80 to 120 days prior to consent. Note: Participant must not have confirmed progression since commencing induction. * Participant must have documented response of PR or VGPR at time of consent. * Participant must have Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1 (participants with ECOG 2 due to pain because of underlying myeloma-associated bone lesions are eligible per investigator's discretion). * Participant must have recovered to ≤ Grade 1 for any nonhematologic toxicities due to prior treatments, excluding alopecia and Grade 2 neuropathy. Exclusion Criteria * Participant with known central nervous system involvement with myeloma. * Participant has non-secretory MM. * Participant has systemic and uncontrolled fungal, bacterial, viral, or other infection. * Participant has history of primary immunodeficiency. * Participant has previous history of an allogeneic hematopoietic stem cell transplantation or treatment with any gene therapy-based therapeutic for cancer or investigational cellular therapy for cancer or B-cell maturation antigen targeted therapy. * Other protocol-defined Inclusion/Exclusion criteria apply.