Rare blood clot risk is far higher from Covid-19 than from vaccines, research suggests

Scientists at University of Oxford find coronavirus markedly increases risk of CVT

The risk of experiencing a rare type of blood clot is far higher for those who catch coronavirus than it is for people who have received three of the main Covid-19 vaccines, preliminary research suggests.

Scientists from the University of Oxford have said the risk of cerebral venous thrombosis (CVT) – a blood clot in the brain also referred to as CVST (cerebral venous sinus thrombosis) – is about eight to 10 times higher after catching the virus than after getting vaccinated with the BioNTech/Pfizer, Moderna or Oxford/AstraZeneca Covid-19 jabs.

The researchers said CVT is more common after Covid-19 than in any of the comparison groups, with 30 per cent of the former cases occurring in the under-30s.

These findings come as the State this week restricted the use of the AstraZeneca vaccine to people aged over 60 following reports of these rare blood clots in people who had received the jab. Many European countries have made similar moves.


The paper from Oxford scientists, which has yet to be peer-reviewed, looked at TriNetX – a US electronic health records database with more than 81 million participants which included information on those who have had Covid-19 as well as people who had received Pfizer or Moderna vaccines.

For the AstraZeneca vaccine, the researchers looked at data from the UK's MHRA as well as the European Medicines Agency (EMA).

Led by Prof Paul Harrison and Dr Maxime Taquet from Oxford's department of psychiatry, the researchers counted the number of CVT cases diagnosed in the two weeks following a diagnosis of Covid-19, or after the administering of the first dose of a vaccine.

Prof Harrison, who is professor of psychiatry and head of the translational neurobiology group at the University of Oxford, said the team had reached two important conclusions, saying: “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 [of such blood clots] is higher than seen 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.”


Based on data from more than 500,000 Covid-19 patients, the experts calculated the occurrence of CVT in such people was 39 in 1 million people.

Among those who had had a vaccine made by Pfizer or Moderna, the occurrence of CVT was about four in 1 million, while for the AstraZeneca vaccine, the occurrence was about five in 1 million after the first dose.

However, the researchers sounded a note of caution on the results, saying that that data comes from multiple sources and is still accruing and the participants have not been adjusted for age.

They also said there is no new data on what is happening after the AstraZeneca vaccine because it is not being used in the US.

Dr Maxime Taquet, also from the translational neurobiology group, said: “It’s important to note that this data should be interpreted cautiously, especially since the data on the Oxford/AstraZeneca vaccine come from UK MHRA monitoring, whereas the other data uses the TriNetX electronic health records network.

“However, the signals that Covid-19 is linked to CVT, as well as portal vein thrombosis – a clotting disorder of the liver – is clear, and one we should take note of.”

The researchers noted also that it was still unclear whether “Covid-19 and vaccines lead to CVT by the same or different mechanisms”, adding that this would be the subject of their ongoing research. – PA