Study links AstraZeneca COVID-19 shot to bleeding disorder ITP

Researchers in Scotland have identified a possible link between the Oxford University/AstraZeneca COVID-19 and a mostly mild and generally treatable bleeding disorder.

The scientists examined real-world public health data from all individuals in Scotland who received either the AZ or Pfizer/BioNTech vaccines up until 14 April and found a possible association between the AZ shot and an autoimmune disorder called idiopathic thrombocytopenic purpura (ITP).

ITP leads to a reduction in blood platelets and is usually mild and self-limiting, with symptoms such as bruising and bleeding episodes, although in very rare cases it can be more serious.

The authors of the study published in Nature Medicine looked at data from 5.4 million people and concluded that ITP was marginally higher among those receiving a first dose of the AZ vaccine than in an unvaccinated control group, with an incidence of 1.13 cases per 100,000.

Patients with ITP tended to be older, with chronic health conditions and in hospital, and almost half of them were also taking medications that could raise the risk of ITP.

Meanwhile, the study found no association between the AZ vaccine and venous thromboembolic (clotting) events – including cerebral venous sinus thrombosis (CSVT) – although there was an increased risk of arterial clotting and bleeding events.

"Given these small increased risks for ChAdOx1, alternative vaccines for individuals at low COVID-19 risk might be warranted when supply allows," they conclude.

The scientists have been quick to point out the limitations of the trial, including a small proportion of people aged under 40 and some people who had already received two doses, as well as limited access to hospital records and the inherent difficult in diagnosing ITP accurately.

In addition, they note that ITP has also emerged as an important complication of COVID-19, with a rate estimated at around 0.34% in hospitalised patients, and it has also been associated with other vaccines including jabs for hepatitis B, measles, mumps and rubella (MMR) and influenza.

Statistician Prof Kevin McConway of the Open University said the trial was well-designed and statistically sound, but pointed out that it is not possible to distinguish between ITP and vaccine-induced thrombotic thrombocytopenia (VITT), a condition that also involves platelets and can lead to serious blood clotting disorders that has been linked to both the AZ and Janssen vaccines.

The risk of ITP of about 11 in every 11 million doses is "very small…even if it turns out that it is caused by the vaccine, and it's tiny compared to the risks arising if someone is not vaccinated and developed COVID-19," he added.

Haematology specialist Prof Adrian Newland of Queen Mary University of London also noted that the background rate in the normal population of ITP in adults is around 60 per million, which suggests there would have been around 535 new cases of ITP in the study population over the course of follow-up.

"It is difficult to be certain whether these are vaccine related or chance associations, although the close association of some to the vaccine suggests that it is likely to be related," said Newland.

Many countries around the world – including the UK – are already recommending that alternative vaccines be used in younger people given the risks of clotting-related side effects.