The research assessed more than 29 million people who were vaccinated with the AstraZeneca and Pfizer jabs, and is the first of its kind to compare the vaccines with risks from coronavirus on such a large scale.
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Risks nine times higher from Covid
The study, published in the British Medical Journal, looked at people aged 16 and over who received their first dose of either the AstraZeneca or Pfizer vaccine in England between 1 December 2020 and 24 April 2021.
Findings suggest that the risk of thrombocytopenia, a condition in which a person has a low count of platelets that help the blood to clot, is almost nine times higher in someone infected with coronavirus than in someone who has had one dose of the AstraZeneca jab.
Researchers estimated that among 10 million people vaccinated with AstraZeneca, this would result in only 107 additional cases of thrombocytopenia in the 28 days after vaccination, compared with 934 in people infected with Covid-19.
The findings come after a coroner concluded this week that award-winning BBC radio presenter Lisa Shaw died due to complications due to the AstraZeneca vaccine.
Ms Shaw passed away in May after developing a vaccine-induced thrombosis and thrombocytopenia, just over three weeks after she received her first vaccine dose.
Dr Martina Patone, a statistician at the University of Oxford, said: “We looked at hospital admission or deaths due to blood clots within 28 days of having either vaccine, and what we found is an increased risk with both vaccines, but also that the risk of blood clots is much higher if you caught Covid-19, either before or after vaccination.”
Link to increased risk of stroke
The study also found a link between the Pfizer vaccine and an increased risk of stroke, although scientists stressed that the risk is more than 10 times higher in those with Covid-19.
There were an estimated 143 extra cases of ischaemic stroke per 10 million people among those vaccinated with Pfizer, compared with 1,699 cases in those with the virus.
For every 10 million people vaccinated with AstraZeneca, there were an estimated seven additional cases of Cerebral venous sinus thrombosis, while there were 20 in people with Covid-19.
For blood clotting in a vein (venous thromboembolism), researchers estimated some 66 excess events per 10 million people vaccinated with AstraZeneca, compared with around 12,614 excess events in those with the virus.
Findings found no associations with blood clots in an artery after either vaccine, although there were some 5,000 excess events per 10 million people infected with coronavirus.
Researchers found no association for AstraZeneca with stroke risk, or Pfizer with increased risk of thrombocytopenia.
Professor Julia Hippisley-Cox, professor of clinical epidemiology and general practice at the University of Oxford, said the increased risks they detected were only for a short time after vaccines, compared to a longer period if infected with the virus.
She explained: “For stroke, with Pfizer it was just 15 to 21 days after vaccination that there was an increased risk.
“And for thrombocytopenia with the AstraZeneca it was eight to 14 days.
“So they were very specific, short periods of time, whereas the associations with infection appeared to be generally over a whole 28-day period after the infection.
“People should be aware of these increased risks after Covid-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2."
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