Rare patients (less than 1 in 100,000) develop thrombosis and thrombocytopenia 5–24 days after vaccination, often with thromboses at unusual sites (cerebral venous sinus; portal, hepatic and splanchnic veins), test strongly positive in PF4/polyanion enzyme immunoassays (EIAs), and show serum-induced platelet activation which is maximal in the presence of PF4. Here, PF4 is linked to Venous thrombosis.