In September, 2020, 6 months after the diagnosis of COVID-19, the patient noted a sore throat, but RT-PCR for SARS-CoV-2 was negative. In the days that followed, her general condition deteriorated because of myalgia, arthralgia, fever of up to 41· 0 C, and lymphadenopathy. Laboratory tests showed an increased C-reactive protein (CRP) concentration of 49 mg/dL (reference value< 0· 5 mg/dL) and leucocytosis of 21· 39× 10⁹ cells per L (reference range 4· 4–11· 3× 109 cells per L). In addition, liver enzymes were elevated, with an AST value of 123 U/L and an ALT value of 165 U/L (reference value< 35 U/L). Moreover, NT-pro-BNP was significantly increased to 3856 pg/mL (reference value< 125 pg/mL) and troponin T to 463 pg/mL (reference value< 100 pg/mL). Finally, systemic inflammation was confirmed by an erythrocyte sedimentation rate of up to 94 mm/h (reference range< 25 mm/h), ferritin values of up to 1771· 8 ng/mL (reference range 15–150 ng/mL), and IL-6 serum concentrations of up to 865· 0 pg/mL (< 8 pg/mL). Laboratory tests for antinuclear antibodies, rheumatoid factor, and diverse infectious agents were negative.

Due to recurrent NSAID-refractory fever episodes with up to 41 C over a period of 3 days, the patient was admitted to hospital. Other clinical symptoms included concomitant evanescent salmon-coloured rashes, episodes of hypotension, and resting tachycardia. Mild pericarditis with a small amount of pericardial effusion and pleural effusions were detected by cardiac MRI. 3 days after admission to hospital, the patient was transferred to an intermediate care unit because of respiratory distress. A chest x-ray revealed pulmonary infiltrates. Vasculitis was ruled out by PET-CT; however, a bilateral basal pneumonia with partly encapsulated pleural effusions was detected. Bronchoscopy with bronchoalveolar lavage revealed an acute bronchitis with distinct collapse phenomena and mild nonpurulent secretion. Analysis of peripheral blood samples did not show any viral or bacterial pathogens. Since an infectious disease or malignancy could be ruled out, AOSD was diagnosed. At this time point, the patient fulfilled all major and minor criteria of the Yamaguchi classification. 1 Treatment with intravenous prednisolone at 50 mg/day was introduced, but did not lead to sufficient improvement resulting in an increase of prednisolone dosage to 100 mg 3 days later. Although the CRP value declined, the clinical situation