Vaccines Carry Far Lower Risk for Rare Blood Clots Than COVID, Study Shows

Peter Russell

April 15, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

The risk of developing cerebral venous thrombosis (CVT) from COVID-19 was "many-fold" higher than from receiving the AstraZeneca/Oxford or the mRNA vaccines from Pfizer and Moderna, researchers have concluded.

A preprint study by the University of Oxford found that from a dataset of over 500,000 COVID patients, CVT would have occurred in 39 per million people.

CVT has been reported to occur in about 5 per million people after a first dose of the AstraZeneca/Oxford vaccine. In over 480,000 people receiving either the Pfizer/BioNTech or Moderna mRNA vaccines, CVT occurred in 4 per million.

The researchers said that compared to the mRNA vaccines, the risk of CVT from COVID-19 was about 10 times greater.

Compared to the Oxford vaccine, the risk of CVT from COVID-19 was about 8 times greater.

The COVID vaccine from Johnson & Johnson, which has been put into limbo because of cases of rare blood clots in patients that received it, is an adenovirus vaccine similar to AstraZeneca's but was not included in this research.

Similar Pattern for PVT

A similar pattern was seen in portal vein thrombosis (PVT) which occurred in 436.4 per million people who had COVID. That compared to 44.9 per million for the mRNA vaccine group, and 1.6 per million for those receiving the Oxford vaccine.

Study author Paul Harrison, professor of psychiatry at the University of Oxford, said in a press briefing that "all the evidence we have is that the risks of COVID are so much greater than whatever the risks of the vaccine might be compared to background".

Commenting on the preprint, Paolo Madeddu, professor of experimental cardiovascular medicine at the University of Bristol, said: "The major issue here is that the comparison showing the higher risk after COVID-19 does not exclude the possibility the pathogenesis is the same and therefore some common denominator should be searched.

"For instance, if the mechanism is the same, one can speculate that the high occurrence in COVID-19 vs. vaccination is because the whole virus is more thrombogenic than the spike protein alone.

"These studies are important but seem to be focused on demonstrating the minor risk of vaccination instead of making efforts to explain the cause of complications, taking advantage of the similarities of the events in the two populations."

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