THE Pfizer vaccine has been linked to a slight, increased risk of stroke in the largest study of its kind so far.

Research involving almost 30million people found that hospital admissions or death from blood clots and a bleeding disorder increased for “short time intervals” after first doses of the Pfizer-BioNTech or AstraZeneca vaccines.

The Oxford vaccine was associated with a slightly increased risk of blood clots in the veins and thrombocytopenia (low platelets) while Pfizer increased the incidence of ischaemic strokes and arterial blood clots.

However, the risks of “most of these events” were substantially higher and more prolonged among those who tested positive for the virus.

A number of European countries including Denmark suspended the use of the AstraZeneca vaccine after a small number of patients developed blood clots. Of the 412 people who developed blood clots up to August 11, 73 died.


The study, which involved several UK universities including Edinburgh, is the first to look at side effects from both vaccines and compare this with Covid outcomes.

Further analysis found that between days 15-21, after the first dose of the Pfizer vaccine, the risk of stroke caused by a blood clot in the brain (ischaemic stroke) was raised by 12%.

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However, for those people with the virus, the risk of stroke was significantly raised from day 0 through to 28 days, with the risk doubled at 15-21 days and overall, was 12 times higher. 

There would be an estimated 1,699 extra cases for every 10 million people after infection, while there would be only 143 extra cases after the first Pfizer jab.

The rate of hospitalisations for thrombocytopenia was 107 people per 10million, 8-14 days after having the AstraZeneca vaccine but was nine times higher (934) among those who tested positive for Covid.

The Oxford vaccine was associated with a slightly increased risk of blood clots in the veins (66 people per 10million) but this increased to 12,614 if they had the virus.

Researchers said the risk of thrombocytopenia associated with the AstraZeneca drug was the same as having other, commonly used vaccines including the flu jag.

Data was obtained for 29,121,633 patients over 16 who received a first dose of either vaccine in England between December 1 2020 and April 24 2021. 

Of the 29,121, 633 participants, 19,608,008 received Oxford-AstraZeneca and 9,513,625 had the Pfizer-BioNTech jag while 1,758 095 people had a positive Covid test.


There was no increased risk of heart attack with either vaccine. 

Only 6% of those who were included in the study had Covid but the risks associated with both vaccination and infection were calculated separately.

More than 90% had the infection prior to vaccination. 

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During the study period, among those vaccinated, 9764 people had a hospital admission related to thrombocytopenia (52 deaths) and 23390 people were admitted to hospital with venous thromboembolism (1871 deaths)

Hospital admission related to arterial thromboembolic events occurred in 89,321 people (6533 deaths); these included 28222 ischaemic strokes (4204 deaths).

Aziz Sheikh, Professor of Primary Care Research & Development and Director of the Usher Institute at The University of Edinburgh and a co-author of the paper, said: “This enormous study, using data on over 29 million vaccinated people, has shown that there is a very small risk of clotting and other blood disorders following first dose Covid-19 vaccination. 

“Though serious, the risk of these same outcomes is much higher following SARS-CoV-2 infection. 

“On balance, this analysis therefore clearly underscores the importance of getting vaccinated.

“What we would advocate, is that there is a clear communication of risks and benefits.”

The study, which is published in the BMJ today, also involved the universities of Oxford, Leicester, Cambridge and Nottingham.

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Dr Richard Francis, Head of Research at the Stroke Association said:  “We have known since early in the pandemic that being infected by the COVID-19 virus has led to strokes in some people and it became apparent during the vaccination rollout that the AstraZeneca vaccine slightly increased the risk of an incredibly rare type of stroke.

"This new study tells us...the risk of having a stroke due to catching COVID-19 before vaccination is much greater than your risk of stroke from having the vaccine."

It comes after a coroner ruled yesterday, that an award-winning radio presenter died due to rare complications from the AstraZeneca vaccine.

Lisa Shaw, 44, lost her life in May, just over three weeks after receiving her first dose, an inquest in Newcastle heard.

The BBC Radio Newcastle host - referred to by her married name, Lisa Eve, during the hearing - began to complain about a headache a few days after the vaccination.

She was transferred to Newcastle’s Royal Victoria Infirmary for specialist treatment - including cutting away part of her skull to reduce pressure but died on May 21, this year.

Newcastle senior coroner, Karen Dilks concluded that she died “due to complications of an AstraZeneca COVID vaccine.”