A 44-year-old male health care worker presented with fevers, fatigue and head “fogginess” with abdominal discomfort and increased bowel frequency 8 days after receiving his first dose of the COVID-19 vaccine (ChAdOx1-S [recombinant])(AstraZeneca). He was previously well, with a past history of depression and was only taking escitalopram. He had no prior thrombosis or exposure to heparin. The low platelet count, 70× 109/L (reference range, 150–400× 109/L), and markedly elevated D-dimer, 114 mg/L (upper limit of normal, 0.5 mg/L), with vague abdominal pains prompted a computed tomography (CT) venogram of the abdomen, which demonstrated thrombosis with complete occlusion of the portal and splenic veins and protrusion of a tongue of thrombus into the superior mesenteric vein (Box 1). CT venogram of the head did not show central venous sinus thrombosis.

The rest of the full blood count and the blood film showed no features of microangiopathic haemolytic anaemia. The prothrombin time, activated partial thromboplastin and fibrinogen levels, liver and renal function tests were all normal. Antiphospholipid antibodies were negative and a heterozygous prothrombin G20210A mutation was identified. Antibodies to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein and the respiratory polymerase chain reaction (PCR) test for coronavirus disease 2019 (COVID-19) were negative, ruling out active COVID-19 infection as a potential thrombosis aetiology.