THE AstraZeneca Covid jag is linked to a rare blood condition characterised by a low platelet count, according to a world-first study involving 1.7 million people in Scotland given a single dose of the vaccine.

Researchers found that there is a very small increased risk that adults given the Oxford-AZ vaccine go on to develop idiopathic thrombocytopenic purpura (ITP).

They estimate that the vaccine is associated with around 11 cases of ITP per million doses - similar to figures seen for the hepatitis B, MMR and flu vaccines, which range from 10 to 30 cases per million doses.

The same pattern was not found for the Pfizer-BioNTech vaccine.

A low number of platelets – blood cells that help prevent blood loss when vessels are damaged – can lead to an increased risk of bleeding or, in some cases, clotting.

READ MORE: Scotland's Covid hospital admissions double 

However, ITP - which can be asymptomatic and is sometimes only detected by chance - is treatable and there were no fatalities associated with it in the study population.

The authors said this was "very reassuring", and stressed that the risk of developing ITP as a result of Covid-19 was even higher and that people should continue to take the vaccine when offered.

The findings, published today in the journal Nature Medicine, were led by scientists at Edinburgh University as part of the ongoing Eave II project which is using anonymised patient data to track the vaccine roll out in real-time and evaluate its impact on the pandemic.

Concerns have previously been raised that the AstraZeneca vaccine increases the risk of patients developing a type of rare blood clot known as a cerebral venous sinus thrombosis (CVST), which prevents blood draining from the brain.

However, the researchers said the tiny number of CVST cases which occurred in Scotland over the study period - just 19 in total - meant it was impossible to reliably evaluate whether there is a genuine link.

Notably, the majority of cases had occurred among unvaccinated individuals.

HeraldScotland: Professor Aziz SheikhProfessor Aziz Sheikh

"If it does exist it's a rare outcome," said Professor Aziz Sheikh, director of Edinburgh University's Usher Institute and study lead for the EAVE II project.

The study is the first of its kind to analyse the post-vaccination incidence of ITP, clotting and bleeding events across an entire national population.

The findings are based on 5.4 million people in Scotland, of whom 1.7 million had had a first dose of AstraZeneca and 800,000 Pfizer by April 14.

READ MORE: Vaccines breaking the link between cases and hospitalisations

Before the pandemic, rates of ITP in the UK were around six to nine cases per 100,000 people, but this has increased as a result of Covid infections.

The AstraZeneca vaccine alone is estimated to be contributing an additional 1.13 cases per 100,000.

The researchers found that cases of ITP in the vaccinated cohort were more likely to occur in patients with pre-existing chronic conditions such as asthma, cardiac problems or diabetes and a median age of 69, whereas unvaccinated cases had an average age of 54 and were generally healthier.

There were comparatively few vaccinated people under-40 in Scotland by April 14, however, due to the age-based prioritisation of the roll out.

HeraldScotland:

The UK's Joint Committee on Vaccination and Immunisation (JCVI) already recommends that healthy adults under 40 be offered an alternative to the AstraZeneca vaccine based on modelling that their chance of developing a blood clot - at around one per 60,000 people for those in their 30s - outweighs their risk of critical illness from Covid infection while the prevalence of the virus is low.

The UK's Medical and Healthcare products Regulatory Agency (MHRA) has previously reported low platelet counts occurring in combination with blood clots in patients given the AstraZeneca vaccine.

In May, it said it was aware of 242 clotting cases and 49 deaths out of 28.5 million doses of the vaccine administered.

READ MORE: Percentage of Covid cases going on to require hospital care 'has halved' since start of 2021

The researchers stress than while the study adds to the evidence linking the AstraZeneca vaccination to blood clots and ITP, a causal association is yet to be definitively proven.

Professor Sheikh added: “This careful analysis of an entire country’s vaccination programme, which involved the study of over 2.5m first dose vaccines, has found a small increase in the risk of ITP, clotting and bleeding events following the Oxford-AstraZeneca vaccine.

"This very small risk is important, but needs to be seen within the context of the very clear benefits of the vaccines and potentially higher risks of these outcomes in those who develop Covid-19.”

Professor Andrew Morris, vice principal of data science at Edinburgh University said the findings were "a terrific example of why access to health data is crucial for vital research that rapidly informs the response to the Covid-19 pandemic".