The risk of developing thrombosis and blood clots is much lower after taking the covid-19 vaccine than when contracting the disease, says the largest study to date on vaccination-related side effects.
The British study, published in the British Medical Journal (BMJ), compared medical data from 29 million people who received the first dose of Pfizer-BioNtech or Oxford-AstraZeneca vaccines between December 2020 and April 2021 with information from nearly two millions of people who have tested positive for the new coronavirus.
Concern about thrombosis has held back the use of the AstraZeneca vaccine, but the study found that while there is a “greater risk” of developing them after the vaccine, it is “much less than that associated with SARS-CoV-2 infection” .
The risk of developing a venous thrombosis is almost 200 times greater with covid-19 (12,614 additional cases out of 10 million people) than with AstraZeneca (66 additional cases).
As for arterial thrombosis, an excess of cases was not registered for either of the two vaccines, but 5,000 additional cases were counted among 10 million people infected with covid-19.
Thus, people with the virus are 11 times more likely to have a stroke (an additional 1,699 cases per 10 million people) than those vaccinated with Pfizer (an additional 143 cases).
“The vast majority of patients are perfectly fine with these vaccines,” the scientist who coordinated the study, Julia Hippisley-Cox, told the BBC, before saying that the “very rare cases” of blood clots should be “put into context”.
The Oxford Professor of Epidemiology also highlighted that the increased risk of developing blood clots is concentrated in more “specific and shorter” periods with vaccines (“from 15 to 21 days after administration” with Pfizer for stroke, “from 8 to 14 days for thrombocytopenia with AstraZeneca”) than after covid-19 infection, when the risk extends “for more than 28 days after infection”.
The study came after many countries – including the UK – decided to reserve the AstraZeneca vaccine for the older population due to fears of thrombosis.
The British Public Health Service estimates that vaccines have saved more than 100,000 lives in the UK, where the pandemic caused 132,000 deaths.
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