The risk of developing rare but serious blood clots from COVID-19 is many times higher than from the AstraZeneca/Oxford or Pfizer and Moderna COVID-19 vaccines, researchers have concluded.

A preprint study by the University of Oxford examined the records of more than 500,000 COVID-19 patients and used that data to estimate that cerebral venous thrombosis blood clots would occurred in about 39 of every 1 million people with COVID-19.

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

The researchers said that compared to the Pfizer or Moderna vaccines, the risk of CVT from COVID-19 was about 10 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 so-called adenovirus vaccine similar to AstraZeneca’s but was not included in this research.

A similar pattern was seen in portal vein thrombosis (PVT) blood clots, which occurred in 436.4 per million people who had COVID. That compared to 44.9 per million for the Pfizer-Moderna vaccine group, and 1.6 per million for those receiving the AstraZeneca 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.”