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Risk of Clots Higher With Covid Than After Vaccine, Study Says

Vials of the AstraZeneca Plc Covid-19 vaccine in Seoul. 

Vials of the AstraZeneca Plc Covid-19 vaccine in Seoul. 

Photographer: SeongJoon Cho/Bloomberg

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 like those developed by Pfizer Inc. and BioNTech SE or Moderna Inc., and five in a million people after AstraZeneca Plc’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.