Prolonged Thrombocytopenia in a Case of MIS-C in a Vaccinated Child.

Multisystem inflammatory syndrome in children (MIS-C) has been extensively described in patients following severe acute respiratory syndrome coronavirus 2 infection. There are now questions about what MIS-C may look like in vaccinated children. Multisystem inflammatory syndrome in children has many clinical and laboratory features in common with other inflammatory disorders including Kawasaki disease and toxic shock syndrome. Rheumatologic conditions can present with similar musculoskeletal complaints and elevated inflammatory markers. Laboratory markers and clinical symptoms of MIS-C usually improve once therapy is begun. We describe a child with persistent thrombocytopenia as an example of variable presentation of MIS-C in vaccinated children. This case report discusses an atypical progression of MIS-C in a vaccinated child with a known prior positive COVID-19 polymerase chain reaction (PCR) test. She presented with nonspecific abdominal pain and fever and was found to have elevated inflammatory markers, lymphopenia, and thrombocytopenia. Intravenous immunoglobulin and steroid treatment failed to induce rapid recovery in her clinical condition or thrombocytopenia. Rheumatologic, hematologic, oncologic, and infectious causes were considered and worked up due to the uncertainty of her case and persistence of pancytopenia but ultimately were ruled out with extensive testing and monitoring. It was key to include a broad differential including viral-induced bone marrow suppression, idiopathic thrombocytopenic purpura, secondary hemophagocytic lymphohistiocytosis, systemic juvenile idiopathic arthritis, and malignancy. The spectrum of MIS-C and response to treatment continues to evolve, and prior vaccination in this child's case complicated the clinical picture further. Additional evaluation of MIS-C in vaccinated cases will permit characterization of the range of MIS-C presentation and response to standard therapy.