The risk of developing blood clots is much lower after vaccination against COVID-19 than when contracting this disease. This is the largest study to date of the side effects of the vaccine. The British study, published in the British Medical Journal (BMJ), compared the medical records of 29 million people who received their first dose of Pfizer-BioNtech or Oxford-AstraZeneca between December 2020 and April 2021 with that of nearly 2 million people who tested positive for coronavirus.
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Door Redactie Online
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The fear of blood clots delayed the use of AstraZeneca’s vaccine. The researchers found that there was indeed an “increased risk” of developing it after vaccination. However, that risk is “much lower than that associated with COVID infection”.
After all, the risk of developing venous thrombosis (phlebitis) is almost 200 times higher from getting COVID (12,614 extra cases in 10 million) than from being vaccinated with AstraZeneca (66 extra cases). With regard to arterial thrombosis, no additional case was found for either vaccine. But an additional 5,000 cases were detected among the 10 million people who had COVID.
More risk of stroke due to corona infection
People infected with the virus are therefore 11 times more likely to have a stroke (1,699 extra cases in 10 million people) than those vaccinated with Pfizer (143 extra cases).
“The vast majority of patients will be fine with these vaccines,” study researcher Julia Hippisley-Cox told the BBC. She added that the “very rare cases” of blood clots should be put in context.
Only temporary risk
The Oxford professor of epidemiology further pointed out that this increased risk of developing blood clots was concentrated over more “specific and short” periods with the vaccines (15 to 21 days after Pfizer administration for stroke, 8 to 14 days for thrombocytopenia with AstraZeneca), where after being infected with COVID-19 the risk persists “more than 28 days after infection”.
The study came about because many countries have decided to reserve the AstraZeneca vaccine for an elderly population, because of fears of blood clots forming.
The UK Health Service (PHE) estimates that vaccines have prevented more than 100,000 deaths in the UK, where the pandemic has claimed 132,000 lives.
