The risk of blood clots among those who’ve been diagnosed with Covid-19 is higher than among those who’ve received vaccines against the disease, according to a new study from the University of Oxford.

Covid patients saw a clot risk of 39 in a million. That compared with four in a million in mRNA vaccines such as those developed by Pfizer and BioNTech or Moderna, and five in a million people after AstraZeneca’s vaccine. Though the analyses for the three vaccines are based on different data sets, making comparisons difficult, the study suggests that the risk of a clot among those with the disease is about eight to 10 times higher than after vaccination.

“All the evidence we have is that risks of Covid are so much greater than whatever the risks of the vaccines might be,” Paul Harrison, professor of psychiatry at Oxford and one of the study’s authors, said. “And unfortunately you might think you’re not going to catch it, but I think everybody would agree there’s a risk of catching Covid in the current climate.”

It was hard to compare the baseline risk of clotting in the general population without Covid because incidence is rare and varies across age groups, the authors said in a media briefing.

Regulators in the UK and EU have placed restrictions on the use of the AstraZeneca vaccine, while shots of a separate inoculation from Johnson & Johnson were placed on hold in much of the world, amid concern over an association with the rare clots. The relative benefits of the AstraZeneca shot, compared with the risk of clots, decrease when doses are given to younger adults, according to an analysis by the UK’s Medicines and Healthcare Products Regulatory Agency.

The authors of the Oxford study counted the number of cerebral venous thrombosis cases diagnosed in the two weeks after Covid diagnosis and after the first dose of a vaccine.

The authors are not associated with the Oxford Vaccine Group, which helped develop the AstraZeneca vaccine.



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