A case of transient monoclonal plasma cell aberrancy following respiratory syncytial virus (RSV) vaccination.

Plasma cell neoplasms include a spectrum of disorders ranging from monoclonal gammopathy of undetermined significance to multiple myeloma, defined by clonal plasma cell proliferation and production of monoclonal immunoglobulins. However, transient monoclonal plasma cell expansions without neoplastic features are rarely documented. Herein, we report a 77-year-old female patient with a history of stage IVa high-grade serous carcinoma of the ovary who was found to have leukocytosis and 22% circulating monoclonal plasma cells during routine preoperative testing, 4 days after receiving the RSV vaccine. Flow cytometry demonstrated lambda-restricted plasma cells expressing CD19+, CD27+, CD38+, CD56-, and CD138+. Serum protein electrophoresis and immunofixation were unremarkable, with a normal κ/λ ratio and absence of M-protein. Repeat flow cytometry 6 days later revealed normalization to polyclonal plasma cells. No clinical or biochemical evidence supported a plasma cell neoplasm, and bone marrow biopsy was postponed. This case underscores the need for clinical awareness of post-vaccination immunologic phenomena that may mimic plasma cell neoplasm. Recognition of such transient aberrancies can prevent unnecessary invasive investigations and broaden understanding of vaccine-induced immune dynamics.