Scientists tie platelet factor 4 to AstraZeneca COVID vaccine-related clots UK researchers have uncovered the novel mechanism behind rare abnormal blood clotting seen in some AstraZeneca/Oxford vaccine recipients, according to a study today in the New England Journal of Medicine. The study involved clinical and lab evaluation of 23 previously healthy patients who experienced blood clots and thrombocytopenia (low platelet counts) 6 to 24 days after receiving the first dose of the AstraZeneca vaccine. Most clots were cerebral venous thrombosis, while some were arterial thrombosis and venous thromboembolisms such as pulmonary embolisms. Twenty-one patients had antibodies for clot-promoting platelet factor 4 (PF4) on enzyme-linked immunosorbent assay (ELISA) before heparin administration. They appeared to be of the immunoglobulin G (IgG) subtype. Functional heparin-induced thrombocytopenia (HIT) testing confirmed the positive PF4 ELISA result in five of seven patients tested.