There are a few rare conditions that potentially increase the risk of TTS. These conditions are: cerebral venous sinus thrombosis (a type of blood clot in the brain) heparin-induced thrombocytopenia (a reaction to a medication called heparin) idiopathic splanchnic thrombosis (a blood clot in abdominal veins) antiphospholipid syndrome with thrombosis TTS is different to these rare conditions. But because there are similarities between TTS and each of these conditions, as a precaution it's recommended that people with a history of any of these conditions have the Pfizer (COMIRNATY) vaccine. The Pfizer vaccine is not linked with TTS. There is no evidence that people with a history of blood clots (other than those described above) are at an increased risk of TTS.