Since the April 14 meeting of the CDC 's Advisory Committee on Immunization Practices [ACIP, click], information has become available about "Vaccine-Associated Prothrombotic Immune Thrombocytopenia" [VAPIT, or the ultimate name and acronym the scientific community ultimately settles on, if any].
Some vaccine adenovirus vectors may induce a rare autoimmune response that appears to associate with platelet counts of 10–20,000/uL and thrombotic events such as pulmonary embolism, cerebral venous sinus thrombosis [CVST] and abdominal splanchnic-vein thrombosis. Reports originated from European observations following administration of the Astra-Zeneca vaccine and later from the US involving the Janssen/Johnson & Johnson product. In addition to thrombocytopenia, marked D-dimer elevation and hypofibrinogenemia are characterstic laboratory findings. The rare events appear mostly in people under 40, more women than men, and their incidence varies widely among parallel reports from various countries. In many cases the autoantibody tests positive for platelet factor 4 [PF4 ] using enzyme immunoassay methods, though negative in the functional serotonin release assay [SRA]. Because anti-PF4 specificity resembles the autoantibody responsible for heparin-induced thrombocytopenia with thrombosis [HIT ], an antithrombotic other than heparin may be indicated. In the European reports there appear to be no relationship to thrombophilia test results, oral contraceptive use, or anti-phospholipid antibodies. I've attached the ACIP meeting PowerPoint summary from Dr. Beth Bell, Work Group Chair, and it also is accessed through the ACIP site.
Thanks to Bob Gosselin and Maureen Smythe for the heads-up on this issue, and watch Fritsma Factor for updates, as this is a developing concern, and indeed, given the rare incidence of VAPIT in comparison to unselected populations and controls, it may turn out not to be vaccine-related at all. Owing to its rarity and Covid's morbility and mortality, worldwide opinion leaders and organizations such as the International Society for Thrombosis and Haemostasis recommend against vaccination "pauses."