• Subjects must satisfy the following criteria to be enrolled in the study:

⁃ Subject is ≥ 19 years of age at the time of signing the informed consent form (ICF).

‣ Subject must understand and voluntarily sign an ICF prior to any study-related assessments/procedures being conducted.

∙ Subject is willing and able to adhere to the study visit schedule and other protocol requirements.

∙ ④ Subjects have documented seropositivity for CMV (CMV IgG seropositvity) at any point. CMV IgG status will be re-checked during screening.

∙ ⑤ Subject has documented diagnosis of MM and measurable disease, defined as any of the following: A. M-protein ≥ 0.5 g/dL by serum protein electrophoresis (sPEP) or B. M-protein ≥ 200 mg/24-hour urine collection by urine protein electrophoresis (uPEP) or C. For subjects without measurable disease in sPEP or uPEP: serum free light chain (sFLC) levels \> 100 mg/L (10 mg/dL) involved light chain and an abnormal kappa/lambda FLC ratio.

∙ \*Patients with extramedullary disease (EMD) only will be allowed to participate if there is a measurable extramedullary lesion. These patients require PET-CT follow-up for response evaluation.

∙ ⑥ Subject has received 3 or more prior lines of antimyeloma therapy. (Note: One line can contain several phases \[e.g., induction, (with or without) hematopoietic stem cell transplant, (with or without) consolidation, and/or (with or without) maintenance therapy).

∙ ⑦ Subject must have received at least one proteasome inhibitor, one immunomodulatory drug, and one anti-CD38 antibody.

• Subject must have documented disease progression during or after their last antimyeloma regimen.

⁃ Subject has an Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1 or 2.

‣ Individual of childbearing potential (IOCBP) must:

∙ A. Have two negative pregnancy tests as verified by the investigator prior to starting study treatment. She must agree to ongoing pregnancy testing during the course of the study, and after the end of study treatment. This applies even if the subject practices true abstinence\* from heterosexual contact.

∙ B. Either commit to true abstinence\* from heterosexual contact (which must be reviewed on a monthly basis and source documented) or agree to use, and be able to comply with, 2 forms of contraception: one highly effective, and one additional effective (barrier) measure of contraception without interruption 28 days prior to starting study treatment, during the study treatment (including dose interruptions), and for at least 90 days after the last dose of letermovir or 4 months after the last dose of elranatamab, whichever is longer.

∙ ⑪ Male subjects must: A. Practice true abstinence\* (which must be reviewed on a monthly basis) or agree to use a condom during sexual contact with a pregnant individual or an individual of childbearing potential while participating in the study, during dose interruptions and for at least 90 days after the last dose of letermovir, even if he has undergone a successful vasectomy.

• True abstinence is acceptable when this is in line with the preferred and usual lifestyle of the subject. \[Periodic abstinence (e.g., calendar, ovulation, post-ovulation methods) and withdrawal are not acceptable methods of contraception\].

⁃ Male subjects must agree to refrain from donating sperm while on study treatment, during dose interruptions and for at least 90 days following last dose of letermovir.

‣ Females must agree to refrain from donating eggs while on study treatment and for at least 90 days after last dose of letermovir.

∙ Subjects must agree to refrain from receiving live vaccines while on study treatment, during dose interruptions and for at least 90 days following the last dose of elranatamab.