Vaccine-induced immune thrombotic thrombocytopenia (VITT) is diagnosed if there is venous or arterial thrombosis with thrombocytopenia caused by antibodies against platelet factor 4 (PF4) that activate thrombocytes and is confirmed by a heparin-induced platelet activation (HIPA) test, occurring 4 to 28 days after vaccination against COVID19. This extremely rare syndrome has been recognized after Astra Zeneca (reporting incidence 0.91 per 100.000) and Janssen vaccins (0.22 per 100.000). Causality with the vaccine is difficult to ascertain if the HIPA test is negative, or if there is severe thrombosis (such as cerebral sinus or splanchnic thrombosis) without thrombocytopenia. Symptoms compatible with thrombosis or bleeding occurring in a specific time window after vaccination should prompt urgent assessment of the thrombocyte count and appropriate diagnostic tests. Heparin-like anticoagulants should be avoided and central laboratory assessment is essential. Early recognition likely improves prognosis of this extremely rare but severe complication of vaccination against COVID-19.