A new study found that cerebral venous thrombosis (CVT) blood clots are far more likely to occur after someone contracts COVID-19 than after getting an mRNA vaccine for the virus.
The Oxford study found the incidence of CVT after contracting COVID-19 was 39 per million people, compared to 4.1 per million people in mRNA vaccines and 0.41 per million in the general population. The findings offer one new data point for medical professionals and potential vaccine recipients in their risk assessments for the virus and its remedies.
Use of the Johnson & Johnson COVID-19 vaccine has been paused in the U.S. for a reported six blood clots in roughly seven million recipients. In Europe, a number of countries have suspended the use of AstraZeneca’s vaccine after discovering a blood clot rate of about ten per million. (RELATED: ‘These Are Very Rare Events’: Doctors Emphasize Low Number Of Blood Clots After Vaccine Pause)
Those two vaccines aren’t mRNA vaccines, and thus weren’t studied by the Oxford group. However, according to their findings, the risk of clotting after contracting COVID-19 is still between four and seven times higher than after getting either the Johnson & Johnson or AstraZeneca vaccine.
Potentially important part of the puzzle: researchers argue risk of CVST (cerebral venous sinus thrombosis—the rare side effect that led to J&J pause) is much higher from COVID *itself* compared to AZ or mRNA vaccines. (preprint, usual caveats, see thread/paper for more context). https://t.co/Dt6woDSyes pic.twitter.com/RzXPlIAi3u
— zeynep tufekci (@zeynep) April 15, 2021
Some have speculated that the clots are linked to the adenovirus vector technology present in those two vaccines — a technology not used in Pfizer or Moderna’s mRNA vaccines. Regulators are still investigating the exact cause and risk level associated with the paused shots, though.
The Oxford study is still in pre-print, meaning it hasn’t yet been peer-reviewed and formally published, and there are some caveats to point out. While the researchers found a rate of 4.1 clots per million in mRNA vaccines, the CDC has only found a real-world incidence of 0.01 per million. That data suggests that, if anything, the researchers estimate that “risk after COVID-19 is approximately 8-10 times that reported for the vaccines” could be understated.
It’s also the case that the reported incidence of clotting is inconsistent, particularly with the AstraZeneca vaccine. While the ten per million number comes from European health regulators, the Australian Technical Advisory Group on Immunization found a rate of around five per million, which is more in line with the rate reported by British regulators. (RELATED: Trump Accuses Biden Administration Of Conspiring With Pfizer To Pause Johnson & Johnson Vaccine For ‘Political Reasons’)
What has been consistent is that the clots seem to be more common in women and younger people. AstraZeneca’s vaccine has not yet been approved for use in the United States, while Johnson & Johnson’s will remain paused for an indefinite period of time pending further investigation.