Study of Pomalidomide and Low Dose Dexamethasone With or Without Pembrolizumab (MK-3475) in Refractory or Relapsed and Refractory Multiple Myeloma (rrMM) (MK-3475-183/KEYNOTE-183)

Inclusion Criteria:

* Has a confirmed diagnosis of active multiple myeloma and measurable disease
* Must have undergone prior treatment with ≥2 treatment lines of anti-myeloma therapy and must have failed last line of treatment (refractory to last line of treatment)
* Prior anti-myeloma treatments must have included an immunomodulatory drug (IMiD) AND proteasome inhibitor alone or in combination and participant must have failed therapy with an IMiD OR proteasome inhibitor
* Has performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Scale
* Female participants of childbearing potential must have 2 negative urine human chorionic gonadotropin tests within 10 to 14 days and within 24 hours prior to receiving study medication
* Female participants of childbearing potential and male participants must agree to use adequate contraception 28 days prior to study start and continuing for up to 28 days after the last dose of pomalidomide (or 120 days after the last dose of pembrolizumab)

Exclusion Criteria:

* Has had prior anti-myeloma therapy within 2 weeks prior to study start and has not recovered (i.e., ≤ Grade 1 or at Baseline) from adverse events due to a previously administered agent
* Has undergone prior allogeneic hematopoietic stem cell transplantation within the last 5 years. (Participants who have had a transplant greater than 5 years ago are eligible as long as there are no symptoms of Graft versus Host Disease \[GVHD\]).
* Has received autologous stem cell transplant (auto-SCT) within 12 weeks before the first infusion or is planning for or is eligible for auto-SCT
* Has received previous therapy with pomalidomide
* Has peripheral neuropathy ≥ Grade 2
* Has a known additional malignancy that is progressing or requires active treatment within the last 5 years (except for basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or in situ cervical cancer that has undergone potentially curative therapy)
* Has a history of (non-infectious) pneumonitis that required steroids or current pneumonitis
* Has received prior therapy with an anti-programmed cell death 1 receptor (anti-PD-1), antiprogrammed death-ligand 1 (anti-PD-L1), anti-PD-L2, anti-CD137, or anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) antibody (including ipilimumab or any other antibody or drug specifically targeting T-cell costimulation or checkpoint pathways)
* Is pregnant or breast-feeding