£1.6m study aims to understand why COVID-19 vaccines can lead to rare instances of blood clotting with low platelets

The vast majority of people who experience a side effect from COVID-19 vaccination have only mild reactions lasting for two or three days. However, in March 2021 reports of small numbers of people being admitted to hospital predominantly after the AZ vaccine with what could potentially be a very rare side effect of vaccination began to emerge. These people had blood clots in the major veins in the brain, abdomen, or elsewhere in the body, but at the same time a low level of platelets – which are responsible for clotting – in the blood.

The researchers, from eleven different institutions, supported by a wide range of collaborators within the NHS and national agencies, will work together to study the mechanisms underlying the occurrence of blood clots with low platelets – known as thrombotic thrombocytopenia syndrome (TTS).  This project is supported by the National Institute for Health Research and backed by £1.6 million of government funding from the Vaccine Taskforce.

Chief Investigator Professor Sir Munir Pirmohamed said: “The combination of blood clots with low platelet levels is very rare, and although it has been reported previously – including before the pandemic – the clusters of cases were unusual and an association with the vaccines was suggested. It is important to note that the vast majority of individuals given the vaccine do not develop TTS – but between 1 in 100,000 and 1 in a million do. We do not yet understand why a vaccine that is safe for almost everyone can cause TTS in particular individuals.

“Our research will help understand why COVID-19 vaccines can lead to TTS in rare cases.”

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