First report of a patient meeting criteria for both multisystem inflammatory syndrome in children and adult onset Still's disease.

Background: COVID-19 is associated with a postinfectious hyperinflammatory disorder, multisystem inflammatory syndrome in children (MIS-C), that shares characteristics with still's disease, known as systemic juvenile idiopathic arthritis (SJIA) in children younger than 16, and adult onset Still's disease (AOSD) in children 16 and older. Both MIS-C and SJIA/AOSD can be complicated by macrophage activation syndrome (MAS), a potentially fatal condition of cytokine storm.

Case Presentation: We present a 16 year-old male who developed quotidian fever, headache, conjunctival injection, sore throat, nausea and vomiting, diarrhea, rash, and symmetrical polyarticular arthralgia/arthritis 4 weeks after exposure to SARS-CoV-2 and 2 weeks after his first vaccination against COVID-19.