The risk-benefit balance seems to be tilting seriously to one side. 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. This British study, published in the British Medical Journal (BMJ), 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 coronavirus.
While fears related to blood clots slowed down use of AstraZeneca's vaccine, researchers found that there was indeed an "increased risk" of developing it after being vaccinated, but that risk was high. “Much lower than that associated with SARS-CoV-2 infection”.
"Very rare cases" of blood clots
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).
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.
Thus, people infected with the virus are eleven times more likely to have a stroke (1,699 additional cases in 10 million people) than those vaccinated with Pfizer (143 additional cases).
“The vast majority of patients will do perfectly well with these vaccines,” study researcher Julia Hippisley-Cox told the BBC, saying that “very rare cases” had to be “put into context”. blood clots.
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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 ".
This study comes as many countries - including the United Kingdom - have decided to reserve the AstraZeneca vaccine for an older population, because of these fears related to the formation of clots.
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.