Vaccine-induced immune thrombotic thrombocytopenia is an extremely rare syndrome characterized by blood clots (thrombosis) and low platelet levels (thrombocytopenia) that has been described in some people after initial vaccination with ChAdOx1 nCoV-19 (AstraZeneca), an adenoviral vector vaccine against the spike protein of SARS-CoV-2. Most of those affected were women younger than 50 years of age. Many of those affected had thromboses at unusual sites such as cerebral venous sinus thrombosis or thrombosis in the portal, splanchnic, or hepatic veins. The New England Journal of Medicine in April 2021 published a report of 39 people affected by this syndrome. The condition was fatal in about 40% of patients.
Cines, Douglas B., and James B. Bussel. "SARS-CoV-2 Vaccine-Induced Immune Thrombotic Thrombocytopenia." Apr. 16, 2021. NEJM <https://www.nejm.org/doi/full/10.1056/NEJMe2106315>.