UK-headquartered pharmaceutical giant AstraZeneca has admitted that in "very rare cases" its COVID vaccine can cause a blood clot related side effect in a recent court filing in London. The AstraZeneca vaccine, also manufactured by the Serum Institute of India, was marketed in India as Covishield. The rare side effect linked to the vaccine is known as Thrombosis with Thrombocytopenia Syndrome or TTS. TTS is a rare but serious condition characterised by blood clotting or thrombosis, combined with low levels of platelets or thrombocytopenia.TTS seems to occur because the body's immune system reacts to the vaccine by making antibodies that attack a protein involved in blood clotting.