Table 1:

Vaccine-induced immune thrombotic thrombocytopenia

Clinical features	Findings
COVID-19 vaccines associated with VITT	ChadOx1 (AstraZeneca), Ad26.COV2.S (Johnson & Johnson)
Incidence	Rare, exact incidence unknown
Possible risk factors	Female sex, age less than 60 years
Onset	5–48 days post vaccination
Typical laboratory findings	Thrombocytopenia, platelet range 6000–344,000/µL, low fibrinogen levels (range 0.3–4.4 mg/dL), elevated D-dimer (range 5000–80,000), positive anti-PF4 antibodies, normal or mildly increased PT, INR, aPTT
Sites of thrombosis	Cerebral veins, deep veins of the leg, pulmonary arteries, and splanchnic vessels
Treatment	Non-heparin anticoagulant agents (argatroban, danaparoid, fondaparinux, or direct oral anticoagulants), IVIG, plasma exchange with plasma but not albumin, high-dose corticosteroids. Preliminary evidence suggest heparin is not harmful and may be used. Avoid platelet transfusions
Proposed mechanism	IgG antibodies bind to platelet Factor 4 (positively charged tetrameric protein), inducing platelet activation and subsequent thromboembolic complications

aPTT, activated partial thromboplastin time; INR, international normalized ratio; IVIG, intravenous immunoglobulin; PT, prothrombin time; VITT, vaccine-induced immune thrombotic thrombocytopenia.