UK researchers have found that a blood clot syndrome after the first dose of the Oxford-AstraZeneca COVID-19 vaccine, known as Covishield in India, carries a high risk of death in the young and healthy, although it's extremely rare.

The researchers examined 220 confirmed and probable cases who presented in UK hospitals between 22 March and 6 June.

The study found that the risk of death was particularly high among patients who presented with a low platelet count, cerebral venous sinus thrombosis, and intracranial hemorrhage.

“It’s important to stress that this kind of reaction to the Oxford/AstraZeneca vaccine is very rare,” said Dr Sue Pavord, lead author of the analysis, was quoted as saying by The Guardian.

However, it can be "devastating" for those who develop the blood clots, she said, adding that it often affects the young and healthy, and has high mortality.

"Our overall case–fatality rate was 23%, which compares favorably with descriptions of earlier, smaller cohorts with a case–fatality rate of 50%."

The researchers defined VITT based on five criteria, including onset of symptoms 5 to 30 days after vaccination, a low platelet count, and documented presence of a blood clot.

The risk of death rose significantly to 73 percent in among patients with platelet counts below 30,000 per cubic millimeter and intracranial hemorrhage, the researchers added.

(With inputs from The Guardian)