An informative article is by Dr. Veronica Hackethal from April. It noted that: "The European Medicines Agency has said that, as of April 20, there have been 287 reports of rare blood clots with low platelets after administration of the AstraZeneca vaccine, eight with Johnson & Johnson, 25 with Pfizer, and five with Moderna. The clots are notable because some have occurred in unusual and deadly locations in the veins that drain the brain (known as cerebral venous sinus thrombosis) and the abdomen (known as splanchnic vein thrombosis)." Note that all the major vaccines were cited.
Consider the problem of very low platelet levels in the body that can cause abnormal bleeding, termed thrombocytopenia, especially deadly brain bleeds. Two processes cause this condition. Platelet clearance is an autoimmune process; the body's immune system is out of control and eats up platelets – this is called immune thrombocytopenia (ITP). Second, is platelet consumption that converts blood platelets into clots in the body.