PF4 and thrombophilia: The mechanism is still under investigation, and it has been suggested that it could be a rare vaccine-related variant of heparin-induced thrombocytopenia because of the presence of antibodies to platelet factor 4.4,5 In a study21 assessing the coagulation profile of 190 healthy individuals vaccinated with either BNT162b2 or AZD1222 researchers found no hypercoagulability following vaccination.