The risk of developing a rare brain blood clot is eight to ten times higher in people infected with COVID-19 than those who get a vaccine, a new study has found.

The study by Oxford University last week reported that the risk of the rare blood clotting known as cerebral venous thrombosis (CVT) following COVID-19 infection is around 100 times greater than normal, several times higher than it is post-vaccination or following influenza.

The study follows investigations into links between the AstraZeneca vaccine and rare blood clots and also looked at those who had a Pfizer or Moderna vaccine. According to the study, four people in one million people experience CVT after getting the Pfizer or Moderna vaccine, versus five in one million people for the AstraZeneca vaccine. In comparison, 39 in one million patients who get COVID-19 develop CVT.

Rollouts of AstraZeneca’s vaccine have been halted or limited in many countries, based on concerns about blood clots.

Led by Professor Paul Harrison and Dr Maxime Taquet from Oxford University’s Department of Psychiatry and the NIHR Oxford Health Biomedical Research Centre, the study examined the health records of 81 million people in the US, looking at the number of CVT cases diagnosed in the two weeks following a diagnosis of COVID-19 and the number of cases occurring in the two weeks after people had their first coronavirus vaccine.

They then compared these to calculated incidences of CVT following influenza, and the background level in the general population. The risk of a CVT from COVID-19 is about 10 times greater than the mRNA and eight times greater than the AstraZeneca vaccine. In addition, 80% of people who developed the clots survived.

Reassuring Findings

Based on US data, the Oxford research team said people being vaccinated should be reassured by the findings.
The study has not been through a final review and is still a work-in-progress, but the researchers say it must be “interpreted cautiously because it is difficult to calculate with certainty how common CVTs are in the general population, partly because of just how rare they are”.

According to Harrison: “We’ve reached two important conclusions. Firstly, COVID-19 markedly increases the risk of CVT, adding to the list of blood clotting problems this infection causes. Secondly, the COVID-19 risk is higher than we see with the current vaccines, even for those under 30; something that should be taken into account when considering the balances between risks and benefits for vaccination”.

Prof Beverley Hunt of Thrombosis UK told BBC news that the mechanisms behind people getting clots after COVID-19 and those experiencing clots after vaccines were likely to be different.

“Patients who are hospitalised with COVID-19 have very pro-thrombotic (sticky) changes in their blood, which persist after they have been discharged. This will lead to an increased rate of blood clots.
“The mechanism for the very rare blood clots and low platelet counts seen after the AstraZeneca vaccine is different. It is associated with an immune response.”