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Judy Jideonwo holds her vaccination card after receiving a dose of the Moderna COVID-19 vaccine on April 7, 2021, at Harborview Medical Center in Seattle.

Ted S. Warren/The Associated Press

A study by researchers at the University of Oxford has found that the risk of developing rare blood clots is eight to 10 times higher in people with COVID-19 than in those who have been vaccinated.

The study of more than 500,000 infected people in the United States calculated that the blood clots, known as cerebral venous sinus thrombosis, occurred in 39 out of one million patients with COVID-19. That compares with five in one million for people who received the Oxford-AstraZeneca vaccine and four in one million for those who got the Pfizer-BioNTech shot.

“There’s no doubt that COVID-19 is a much greater risk of this than are any of the vaccines,” said Paul Harrison, a professor of psychiatry at Oxford who co-authored the study.

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Dr. Harrison told a news media briefing on Thursday that the team conducted the research to address the growing controversy over reports that the AstraZeneca vaccine has been linked to cases of CVST, a sometimes fatal condition that occurs when clots form in veins that drain blood from the brain. The Oxford researchers estimated that the mortality rate of people with CVST was 20 per cent.

Denmark recently suspended all use of the AstraZeneca vaccine and several countries, including Canada, have restricted it to older people after reports showed the condition was more common in younger age groups. Regulators in the U.S. have also paused administering the Johnson & Johnson vaccine after six female recipients, all under 50, developed the clots. One woman died.

The Oxford researchers said they hope that their findings will help people better understand the risks of getting COVID-19. “We’ve tried to turn the debate around and put it back on to COVID because even though [CVST] is rare, it appears to be significantly commoner than having the vaccine,” Dr. Harrison said. “We think that’s quite important in helping both regulators and the public understand better the risk-benefit question, which of course is at the heart of what the policy-makers and regulators are trying to do to get it right for the vaccines.”

The researchers used a data set from the TriNetX analytics network, which includes 62 health care organizations in the U.S. comprising 81 million patients.

The study looked at 513,284 cases of COVID-19 and examined how many people developed CVST. They compared that to patients who had the flu and those who had been vaccinated against COVID-19 with the Pfizer vaccine. They drew on data from the European Medicines Agency to compare the findings to cases of clots among people who have received the AstraZeneca vaccine, which has yet to be approved in the U.S.

Of the total number of COVID-19 cases the study examined, 20 people were diagnosed with CVST within two weeks of infection. That equates to 39 out of a million, the researchers said. Of the 20 patients, 10 were under the age of 40, and four people died.

The researchers also looked at 331,503 people who had received the Pfizer vaccine and found that just two had developed CVST, which equates to 4.1 per million. The EMA has also reported 169 cases of the clots out of 34 million vaccinations with the AstraZeneca jab, or five per million.

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The findings concluded that compared with receiving the Pfizer vaccine, the risk of developing CVST from COVID-19 is about 10 times greater. Compared with getting the AstraZeneca vaccine, the risk was roughly eight times higher.

By contrast, none of the people who had the flu developed CVST, and the rate of these blood clots across the entire dataset was 0.41 per million. That meant that the risk of contracting CVST was 100 times higher in people with COVID-19 than in the general population, although the researchers cautioned that the base rate for CVST is difficult to calculate.

The study also examined cases of portal vein thrombosis, or PVT, a type of clotting that develops in vessels that take blood to the liver. Cases of PVT are also rare, but there have been some reports in people who have been vaccinated.

The researchers found 436.4 PVT cases per million in those infected with COVID-19 and 44.9 per million in those who had been vaccinated with the Pfizer shot. The study did not have similar data for the AstraZeneca vaccine.

Health columnist André Picard answers reader questions about COVID-19 variants, how effective the various vaccines are and the impact of on-again, off-again lockdowns. The Globe and Mail

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