Thrombosis with thrombocytopenia syndrome is a very rare side effect of the AstraZeneca COVID-19 vaccine. The AstraZeneca COVID-19 vaccine is no longer available in Australia, but it was available until recently. In Australia, symptoms of TTS have occurred between 4 and 42 days (most commonly 4 to 30 days) after vaccination with the AstraZeneca vaccine. The risk of TTS appears to be higher in people aged under 60 years of age. The blood clots can occur in different parts of the body, such as the brain (called cerebral venous sinus thrombosis, or CVST) or in the abdomen (idiopathic splanchnic vein thrombosis). The mechanism that causes TTS is not fully understood, but it is similar to heparin-induced thrombocytopenia (HIT), a rare reaction to a medication called heparin. TTS is different from general clotting conditions such as deep vein thrombosis (DVT) or pulmonary embolism (PE). Researchers are still working to understand TTS. One theory is that, in rare cases in some people, the AstraZeneca vaccine might produce antibodies that react with platelets — making them ‘stick together’. (Antibodies are the proteins made by your immune system to fight off foreign substances.) This can cause blood clots, which stops platelets (and blood) from circulating through the body. It’s important to note that only a link has been established. The AstraZeneca vaccine has not been proven to cause TTS. Blood clots are common and not all types of blood clot that occur after vaccination will be linked to the vaccine.