What is TTS? The rare blood clot triggered by AstraZeneca's Covid vaccine

AstraZeneca, the British-Swedish pharmaceutical company, recently admitted for the first time in court documents that its Covid-19 vaccine may lead to side effects in “very rare cases.”

The vaccine, developed by AstraZeneca and Oxford University, could cause a rare side effect called Thrombosis with Thrombocytopenia Syndrome (TTS).Â

This is a result of an ongoing legal battle initiated by Jamie Scott, who suffered a permanent brain injury after receiving the AstraZeneca vaccine in April 2021.Â

The Covid-19 vaccine by the company was sold globally under the brand names Covishield and Vaxzevria among others.

According to the court documents submitted to the UK High Court, the vaccine in rare circumstances leads to TTS.Â

WHAT IS TTS?

Thrombosis with Thrombocytopenia Syndrome (TTS) is a serious and life-threatening adverse event.    Â

Also known as Vaccine-induced prothrombotic immune thrombocytopenia (VIPIT) or vaccine-induced immune thrombotic thrombocytopenia (VITT), TTS was a rare syndrome reported in people who received a Covid-19 adenovirus vector-based vaccine.Â

The syndrome causes major blood clots, also called thrombosis formation combined with thrombocytopenia (low platelet count) which would lead to loss of blood flow in a particular region of the body.

Blood clots can happen in different locations, like the brain or the abdomen.

The Covid-19 vaccine by the company was sold globally under the brand names Covishield and Vaxzevria among others. (Photo: Getty Images)

The symptoms of the syndrome, which include severe or persistent headaches, blurred vision, shortness of breath, chest pain, leg swelling or persistent abdominal pain, typically show 4 to 42 days after vaccination.Â

According to the US Centre for Disease Control and Prevention (CDC), TTS is classified into two tiers based on the location of the blood clot and the severity of the symptoms.Â

Tier 1:

• Uncommon site of thrombosis (like the brain-cerebral venous sinus thrombosis or the gut such as splanchnic vein, associated with bowel ischaemia and surgery, portal vein or other rare venous and arterial thromboses)
• Thrombosis in more common locations (deep vein thrombosis or pulmonary embolism)
• Platelet count is less than 1,50,000 per microlitre
• A positive anti-PF4 ELISA result is supportive, but not required for diagnosis.

Tier 2:

• Common sites of thrombosis include leg or lungs (venous thromboembolism, deep vein thrombosis, pulmonary embolism)
• Platelet count is less than 1,50,000 per microlitre
• A positive anti-PF4 ELISA result is required.

As per CDC, Tier 1 TTS carries a higher risk of mortality and morbidity than Tier 2. Tier 1 could be more common among younger age groups.