Lessons of the month 3: Haemophagocytic lymphohistiocytosis following COVID-19 vaccination (ChAdOx1 nCoV-19).

A 36-year-old woman presented to hospital 9 days after receiving her first dose of the ChAdOx1 nCoV-19 vaccine with fever, myalgia and a sore throat. She was previously fit and well with no prior vaccine reactions.There was no clinical response to initial treatment with intravenous (IV) antibiotics. Microbiology tests including for COVID-19 were negative. At day 5, she developed pleuritic pain and a pericardial rub. Echocardiography and subsequent cardiac magnetic resonance imaging showed evidence of constrictive pericarditis. Computed tomography revealed gross hepatomegaly and moderate splenomegaly. Blood tests showed raised inflammatory markers, deranged clotting, low platelets and a marked hyperferritinaemia.A presumptive diagnosis of a multi-system inflammatory disorder secondary to recent COVID-19 vaccination was made and high-dose IV methylprednisolone initiated. Following a high 'H score' of 70%-80% a diagnosis of secondary haemophagocytic lymphohistiocytosis (HLH) was made. She was treated with IV immunoglobulin with subsequent clinical response.HLH is a rare syndrome of acute and rapidly progressive systemic inflammation characterised by cytopenias, excessive cytokine production and hyperferritinaemia. The adult form has multiple triggers, including recent vaccination. This case prompts awareness among clinicians of HLH as a rare complication of COVID-19 vaccination but should not discourage individuals from vaccination. © Royal College of Physicians 2021. All rights reserved.