Do COVID-19 vaccines cause blood clots – if so, which vaccines?
Yes, but very rarely. COVID-19 vaccines are exceedingly safe.
Blood clots (deep vein thrombosis, pulmonary embolism, arterial thrombosis, cerebral venous sinus thrombosis) associated with thrombocytopenia (a low platelet count) can occur extremely rarely, 5-30 days following vaccination with COVID-19 vaccines (predominantly AstraZeneca and Janssen/Johnson & Johnson).
This is known as ‘vaccine-induced immune thrombotic thrombocytopenia (or VITT)’. The mechanism is similar to autoimmune ‘heparin-induced thrombocytopenia’ (autoimmune HIT). Heparin is a drug used to thin the blood.
There are no known risk factors. People with risk factors for venous or arterial thromboembolism (or a history of thromboembolism or HIT) are not more likely to get VITT.
