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People queue to receive a dose of the Pfizer/BioNTech COVID-19 vaccine at a mobile vaccination centre set up in a bus parked outside Premier League club Newcastle United's St James's Park football stadium in Newcastle, United Kingdom on Aug. 15, 2021.LINDSEY PARNABY/AFP/Getty Images

A study by researchers in Britain has found that people infected with COVID-19 are at least nine times more likely to develop potentially deadly blood conditions compared with those who have received either the Oxford-AstraZeneca or the Pfizer-BioNtech vaccines.

The study, led by a team at the University of Oxford, is the largest of its kind and is set to be published in the British Medical Journal on Friday. It involved analyzing medical data from 29.1 million people in England who received one dose of vaccine from Dec. 1, 2020, to April 24, 2021. The scientists also looked at medical records for 1.8 million people who tested positive for COVID-19 during that period.

The researchers zeroed in on three main side effects in both patient groups: clotting in the veins and arteries as well as reports of low platelet counts which can cause heavy bleeding. All of the conditions are rare but other studies have shown that the mortality rate can range from 22 per cent to 73 per cent depending on the severity of the condition.

Blood-clot risk higher for COVID-19 patients than for the vaccinated, Oxford researchers say

Researchers confirm direct link between AstraZeneca’s COVID-19 vaccine and rare blood clots, but say virtually no incidents since age restrictions

Based on results from the study, researchers estimated that if 10 million people were vaccinated with the AstraZeneca jab, there would be 107 cases of low platelet counts. However, if 10 million people became infected with COVID-19, 934 would develop low platelet counts.

The figures were even more striking for vein and artery clots. The researchers calculated that there would be 66 cases of vein clots for every 10 million vaccinations with AstraZeneca and virtually no cases of clots in arteries. In contrast, they estimated 12,614 cases of vein clots for every 10 million people infected with COVID-19 and 5,573 cases of clots in the arteries. They had no estimated cases for the Pfizer jab.

The risks of developing all three side effects was highest within two weeks of inoculation with the AstraZeneca vaccine. However, for those infected with COVID-19, the high-risk period lasted for at least 28 days.

“Whilst there are small risks associated with each of the vaccines, these risks are far lower than actually getting SARS-CoV-2 itself,” said Julia Hippisley-Cox, a professor of clinical epidemiology at the University of Oxford.

“What we would all advocate is that there is a clear communication of risks and benefits of vaccines,” added Aziz Sheikh, a professor of primary care research and development at the University of Edinburgh who also worked on the study. “What’s new here is [the study] is contextualising this with reference to risks encountered to if you actually get the infection. I don’t think we’ve had that discussion transparently before this.”

Concerns about clotting and other blood conditions emerged in March after health officials in several countries reported multiple cases of cerebral venous sinus thrombosis, or CVST, in people who had been vaccinated with AstraZeneca. CVST is a sometimes fatal condition that occurs when clots form in veins that drain blood from the brain. The vaccine had also been tied to other clotting events that were coupled with low platelets. Around 40 people in Canada have developed signs of vaccine-induced immune thrombocytopenia and thrombosis, or VITT, and five have died.

Several countries, including Canada, have limited the use of the AstraZeneca vaccine to people older than 40 or 50.

The Oxford study confirmed other research that has found a higher risk of developing CVST after vaccination with the AstraZeneca jab.

A different study released last month by a group of haematologists in Britain examined 220 cases of VITT and found that they were all linked to a first dose of the vaccine. They also noted that there had not been any VITT incidents in Britain in the past four weeks and only a handful since May, when the government stopped using the shot for people under the age of 40.

The Oxford group found that there was an increased risk of CVST from both the AstraZeneca and Pfizer vaccines. They noted that the previous research relied on data from Scotland where the Pfizer vaccine has not been used as much as in England. They also found a slightly higher risk of ischaemic strokes, or blood clots in arteries in the brain, in people who had the Pfizer jab.

Peter English, past chair of the British Medical Association’s public health medicine committee, said the study raised new questions about vaccination.

“One of the risk assessments that regulators – and people considering vaccination – have to make is not just ‘are you more likely to have this adverse event after the vaccine,’ but ‘are you more likely to have the disease the vaccine is intended to prevent and have this adverse reaction as a result of the disease than if you get vaccinated?’,” he said. “This paper shows clearly that you are far more likely to suffer these adverse events, which are very rare following vaccination, if you get COVID-19.”

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