Risk of rare blood clots far higher if you catch Covid than if you take vaccine, study suggests

The risk of experiencing a rare type of blood clot is far higher for those catching coronavirus than it is for people who have been vaccinated against Covid-19, 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 around eight to 10 times higher after catching the virus than getting vaccinated with 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% of these cases occurring in the under-30s.

These findings come as many European countries have restricted the use of the AstraZeneca jab in younger people following reports of these rare blood clots.

UK medicines regulator the MHRA (Medicines and Healthcare products Regulatory Agency) has said benefits of the AstraZeneca vaccine continue to outweigh any risks, but added that under-30s would be offered an alternative jab.

Many countries have limited the use of the AstraZeneca vaccine.

Up to March 31, the MHRA received 79 reports of blood clots accompanied by low blood platelet count, all in people who had their first dose of the vaccine.

Of these 79, 44 were cases of CVT.

The paper from Oxford University 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 MHRA as well as the European watchdog the European Medicines Agency (EMA).

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

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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 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 was 39 in a million people.

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

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However, the researchers sounded a note of caution on the results, saying 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.”

They 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.