The risk of developing blood clots is much lower after being vaccinated against Covid-19 than by catching this disease, said Friday the largest study to date on the side effects of the vaccine.

Published in the British Medical Journal (BMJ), this British study compared the medical data of 29 million people who received their first dose of Pfizer-BioNtech or Oxford-AstraZeneca between December 2020 and April 2021, with that of almost 2 million people tested positive for the coronavirus.

Study of nearly 30 million people in England vaccinated against # COVID19 published @bmj_latest finds risk of stroke and other adverse events was substantially higher and more prolonged after Covid-19 infection than after # Covidvaccination.https: //t.co/VnKakz8l3u#CovidVaccine pic.twitter.com/r3tlAMeg1E

- MRC Epidemiology Unit (@MRC_Epid) August 27, 2021

While many fears related to the risks of atypical thrombosis slowed the use of AstraZeneca vaccine, researchers found that there was indeed an "increased risk" of developing it after being vaccinated, but that the latter was actually "much lower than that associated with infection with SARS-CoV-2".

A risk almost 200 times higher by catching the Covid-19

The risk of developing venous thrombosis (phlebitis) is almost 200 times higher by catching the Covid (12,614 additional cases out of 10 million) than by being vaccinated with AstraZeneca (66 additional cases), the study concludes.

Regarding arterial thrombosis, no excess case was found for either vaccine, but 5,000 additional cases out of 10 million people were observed in those who had Covid.

People affected by the virus are therefore eleven times more likely to be subject to a stroke (1,699 additional cases in 10 million people) than those vaccinated with Pfizer (143 additional cases).

Put the "very rare cases" of blood clots in context

"The vast majority of patients will be perfectly fine with these vaccines," study researcher Julia Hippisley-Cox told the BBC, saying that the "very rare cases had to be" put into context. ┬╗Blood clots.

The professor of epidemiology at Oxford further pointed out that this increased risk of developing clots was concentrated over more "specific and short" periods with the vaccines ("15 to 21 days after administration" of Pfizer for stroke, " 8 to 14 days for thrombocytopenia with AstraZeneca ") only after catching Covid-19, where the risk continues" for 28 days after infection ".

The study comes as many countries - including the UK - have decided to reserve the AstraZeneca vaccine for an older population, over concerns about clot formation.

The English Health Service (PHE) estimates that vaccines have prevented more than 100,000 deaths in the UK, where the pandemic has claimed 132,000 lives.

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