We describe a fatal case of multisystem inflammatory syndrome in an adult with onset 22 days after a second dose of mRNA coronavirus disease vaccine. Serologic and clinical findings indicated severe acute respiratory syndrome coronavirus 2 infection occurred before vaccination. The immunopathology of this syndrome, regardless of vaccination status, remains poorly understood.A multisystem inflammatory syndrome in children (MIS-C) and adults (MIS-A) occurring after coronavirus disease (COVID-19) has been identified; onset is ≈4–6 weeks after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection (1–3). A case definition for MIS-A has been developed by the Centers for Disease Control and Prevention (CDC) (4); MIS-A after vaccination is rare and remains of great clinical and public health interest (5). We report a case study and histopathologic findings from a fatal MIS-A case after SARS-CoV-2 infection and subsequent complete COVID-19 vaccination. The Patient The patient was a healthcare worker in his 30s with no notable medical history. In December 2020, he experienced mild COVID-19–like illness symptoms, including fatigue and loss of taste and smell. He did not undergo testing for SARS-CoV-2 at that time and was unaware of the need for isolation. Six days after onset of COVID-19–like symptoms, and when fully recovered, the patient received the first dose of Pfizer/BioNTech (https://www.pfizer.comExternal Link) mRNA COVID-19 vaccine. He received the second dose 20 days later. After the second dose, he reported fatigue and malaise, which resolved within 2 days.Twenty-two days after receiving the second dose of the COVID-19 vaccine, he had onset of new fever, malaise, headache, and odynophagia. He was examined by an outpatient medical provider. Diagnostic testing was notable for a negative COVID-19 test by reverse transcription PCR (RT-PCR), negative rapid influenza antigen, and negative rapid antigen detection for group A Streptococcus. Four days later, the patient visited an emergency department because of worsening symptoms. Assessment of vital signs revealed a temperature of 37.2°C, heart rate 113 beats/min, and blood pressure of 117/66 mmHg. Physical examination identified right-sided cervical lymphadenopathy, marked bilateral conjunctival erythema, and a faint papular rash on the pelvis and left flank. Laboratory testing revealed a peripheral-blood leukocyte count of 11,000 cells/μL, 93.5% segmented neutrophils, and thrombocytopenia with a platelet count of 110,000/μL (Table). Portable chest radiograph results were without notable findings.On hospital day 2, the patient remained febrile and tachycardic (heart rate 90–135 beats/min) and had a blood pressure of 92/56 mmHg. Diagnostic evaluation revealed a negative SARS-CoV-2 RT-PCR test but a positive serologic test for SARS-CoV-2 nucleocapsid IgG. Additional diagnostic tests were conducted (Table). Inflammatory markers showed elevated C-reactive protein at 284.0 mg/L, serum ferritin at 1434.9 ng/mL, and troponin-I at 18.0 ng/mL. On the evening of hospital day 2, the patient received 75 g of intravenous immune globulin (IVIG).Early morning