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A National Health Service agent prepares to administer the Oxford / AstraZeneca Covid-19 vaccine on March 21 in London. Dinendra Haria / SOPA Images / LightRocket / Getty Images

Oxford University researchers have found that the overall risk of a rare type of blood clot is low in people infected with Covid-19, but higher than in people who have received the three UK-approved vaccines from AstraZeneca, Moderna and Pfizer.

The study, which was preprinted to a scientific journal on the Oxford website the Thursday prior to publication, says there is a risk of cerebral venous thrombosis, or CVT – also known as cerebral venous sinus thrombosis or CVST – after a Covid-19 infection “100 times higher than normal and several times higher than after vaccination or influenza” in all age groups.

“Covid-19 significantly increases the risk of CVT and adds to the list of blood clotting problems caused by this infection,” said Paul Harrison, professor of psychiatry and head of the translational neurobiology group at Oxford University.

Oxford University, which developed the AstraZeneca vaccine, said the research came from a separate part of the university and was not affiliated with the vaccine team. The data used come from external sources, in particular the European Medicines Agency.

Compared to the risk of blood clots from the three vaccines, the risk of infection is “between 8 and 10 times higher and about 100 times higher for infection compared to baseline,” Oxford said in a press release. According to the study, the risk of CVT due to Covid-19 infection is about ten times higher compared to the mRNA vaccines – Pfizer and Moderna. Compared to AstraZeneca, the risk of a CVT from Covid-19 is about eight times higher. The Johnson & Johnson Covid-19 vaccine was not included in the analysis.

Using a network of electronic health records with over 500,000 Covid-19 positive cases, 489,871 vaccinated cases, and 172,724 influenza cases, the study found that 30% of CVT cases occurred in the under 30 age group who were most susceptible to blood clots are threatened.

“Given the balance between risk and COVID-19 risk, it’s higher than current vaccines, even for those under the age of 30. This should be taken into account when considering the balance between the risks and benefits of vaccination, ”added Harrison.

Dr. Maxime Taquet of the Translational Neurobiology Group in Oxford and co-author of the study warned that data are still available. Researchers also have yet to determine if Covid-19 and vaccines lead to CVT in the same way, she said.

Experts noted that CVT is so rare that there is limited data even before the pandemic, and data and data sources around the Covid-19 vaccines are inconsistent and limited.

“Overall, the main finding is that these CVT events are very rare in Covid-19 patients and people who have had one of the vaccines – some in a million people – but much less common in people who have had a vaccine than in people with Covid-19, ”said Kevin McConway, Professor Emeritus of Applied Statistics at the Open University, in a comment to the Science Media Center in the UK. “The researchers don’t claim that vaccines don’t increase the risk at all compared to the risk in people who haven’t been vaccinated and who haven’t had Covid-19 either – but they do say that the risk of CVT in people who had Covid-19 , equal is about 100 times the risk in the general population. “

Some background information: European and UK drug regulators last week announced a “possible link” between the AstraZeneca vaccine and rare cases of blood clots. The UK announced it would offer an alternative vaccine to people under the age of 30. Other countries have followed suit and either only offer people over a certain age or are like Denmark and Norway and are scrapping the vaccine entirely. While regulators advised the public to look out for signs of blood clots, they said the benefits of the shot were still worth the risk. The AstraZeneca vaccine is not approved for use in the United States.

Six reports of similar coagulation events following vaccination with the Johnson & Johnson single-dose vaccine prompted the U.S. Centers for Disease Control and Prevention and the U.S. Food and Drug Administration to recommend a pause in vaccine administration, to allow further investigation.

Six women between the ages of 18 and 48 had developed cerebral venous sinus thrombosis, a clot in the area of ​​the brain that collects and drains deoxygenated blood. Blood thinners, the typical treatment for blood clots, should not be used in such cases. The six reported cases were among more than 6.8 million doses of the Johnson & Johnson vaccine administered in the United States.

The EU, which relies heavily on the J&J vaccine to help delay vaccination, has also stopped using the shot. The European Medicines Agency is expected to announce a decision on the administration of the Johnson & Johnson vaccine next week.

The WHO said Thursday, “Currently the risk of developing blood clots is much higher for someone with COVID-19 than for someone who has taken the AstraZeneca vaccine.” WHO Regional Director for Europe Hans Kluge reiterated his recommendation of the AstraZeneca vaccine for all eligible adults, calling it “effective in reducing COVID-19 hospitalization and preventing deaths”.