This study tests a new medicine called mRNA-2808 for people with a type of blood cancer, multiple myeloma, that has come back or didn't respond to treatment. The study checks if mRNA-2808 is safe and how people feel taking it. mRNA stands for messenger ribonucleic acid, a special substance in our body that helps make proteins. Relapsed means cancer has returned, and refractory means it hasn't responded to past treatments.

• Participants need to have tried other treatments, including a proteasome inhibitor (a drug that blocks protein breakdown), IMiD (immunomodulatory drug), and an anti-CD38 monoclonal antibody (a special protein that fights cancer cells).
• People can't join if they have cancer in the brain, certain recent treatments, or specific blood conditions.
• The study may have other rules for joining, and participants will be informed about risks and benefits.

Participating involves regular doctor visits and tests to monitor health and how the body responds to mRNA-2808. It's important to discuss with a healthcare provider to see if this study is a good fit.