AstraZeneca has admitted its COVID-19 vaccine, sold under the brand names Covishield and Vaxzevria among others, can cause Thrombosis with Thrombocytopenia Syndrome (TTS) in very rare cases. It occurs when a person has blood clots together with a low platelet count. It's also referred to as 'vaccine-induced immune thrombotic thrombocytopenia'. The vaccine is no longer administered in the UK.