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Clotting problems, which have been called by many different names such as thrombosis with thrombocytopenia syndrome (TTS), have NOT been observed in association with mRNA Covid vaccines. However, they have been a very rare occurrence with the Janssen and AstraZeneca Covid vaccines among young women (about 1 case of TTS per 50,000-100,000 vaccinated women of age 18-49 years); among women older than 50 and men the occurrence is even more rare. The cause is not completely certain but there is evidence that the clotting reaction may be associated with some people having antibodies against “Platelet Factor 4” (PF4). It is not known what is triggering these anti-PF4 antibodies but it is thought to be very similar to a very similar clotting reaction to heparin (heparin-induced thrombocytopenia and thrombosis, HITT). A well-thought article in Nature discusses what is known thus far about TTS with Covid vaccines.