Platelet factor 4

Thrombosis

Immunology

Immune system

Vaccination

Internal medicine

Heparin-induced thrombocytopenia

Medicine

Antibody

Platelet

Heparin

Thrombosis and Haemostasis

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare side effect of two adenoviral vector vaccines, ChAdOx1 nCoV-19 (AstraZeneca) and Ad26.COV2.S (Johnson & Johnson/Janssen); it is caused by platelet-activating immunoglobulin G (IgG) that recognizes platelet factor 4 (PF4), as shown by positive testing by PF4/heparin-enzyme-linked immunosorbent assay (PF4-H-ELISA) in combination with PF4-enhanced washed platelet activation assays (PF4-PAA).[1] [2] [3] Clinical presentation is heterogeneous,[4] with some patients presenting without overt thrombosis (VITT with isolated thrombocytopenia) or with severe headache.[3] [5] [6] Few data exist regarding long-term decline in PF4-dependent antibodies[7]; in addition, there is uncertainty about timing and safety of subsequent booster vaccination after an episode of VITT.

Heterogeneity of Vaccine-Induced Immune Thrombotic Thrombocytopenia after ChAdOx1 nCoV-19 Vaccination and Safety of Second Vaccination with BNT162b2