Coronavirus: Study finds CVST blood clot risk is 10 times higher after Covid than after vaccination

People are up to 10 times more likely to develop a brain blood clot after catching Covid than after getting a vaccine, a study has found.

Cerebral venous sinus thombrosis is the condition that has spooked regulators by appearing in recipients of the AstraZeneca or Johnson & Johnson jabs.

But researchers claim the risk of developing the complication is significantly higher after getting coronavirus than it is after receiving any of the Covid jabs. 

The benefits of getting vaccinated are far higher than the risks, they insist, because the chance of getting a clot is still vanishingly rare and mass vaccination will protect millions of people – both those who get the jabs and the people around them.

Oxford University scientists studied data from the US to work out how often people were diagnosed with CVST after testing positive for coronavirus.

They estimated the rate was 39 cases per million people – 0.0039 per cent, or one in every 25,641.

The rate for people who had Pfizer or Moderna’s vaccine was about four in a million – 0.00039 per cent or one in 250,000.

And, based on European data, they said the risk after AstraZeneca’s jab appeared to be roughly five in a million – 0.0005 per cent or one in 200,000.

‘The key message is that the risk of this particular event is actually much lower than if you get Covid or someone else gets Covid,’ said Dr John Geddes of the university’s biomedical research centre.

The study comes after another Oxford professor not involved with the research, Sir John Bell, said today that the blood clot risk after vaccination was ‘trivial’.  

The Oxford study’s calculations suggested the rate of CVST is 39 in a million among people who have tested positive for Covid-19, compared to between four and five per million after vaccination

The risk of portal vein thrombosis, a type of blood clot in the liver, was also significantly higher after Covid-19 than after vaccination, the data suggested

The risk of portal vein thrombosis, a type of blood clot in the liver, was also significantly higher after Covid-19 than after vaccination, the data suggested

Experts say the risks of not getting vaccinated against Covid are far greater than any risk of having the jab (Pictured: A man being vaccinated in London yesterday, April 14)

Experts say the risks of not getting vaccinated against Covid are far greater than any risk of having the jab (Pictured: A man being vaccinated in London yesterday, April 14)

The study looked at the risk of developing a CVST clot within two weeks of either being diagnosed with Covid or the flu, or taking the Pfizer or Moderna vaccine.

It involved records from 500,000 people, most of them in the US.

No new data were collected about the AstraZeneca vaccine, which is the main one concerning drug regulators, because it isn’t used in the US. 

The purpose of the study was to work out the risk of CVST after Covid, which isn’t well understood.

The study found that the rates of the clots in people who had caught Covid were significantly higher than in people who had been vaccinated.

Pfizer or Moderna’s jabs, which are based on the same synthetic mRNA technology, had a rate 10 times lower than in the Covid patients.

And European Medicines Agency data suggests the risk for AstraZeneca’s jab, which contains a live cold virus, was eight times lower.

They found that, for comparison, nobody developed CVST within two weeks of a flu diagnosis. 


Cerebral venous sinus thrombosis (CVST) is an extremely rare  type of blood clot in the brain.

It may also be abbreviated to CSVT or just CVT – they are all basically the same. 

It occurs when the vein that drains blood from the brain is blocked by a blood clot, resulting in potentially deadly bleeding or a stroke.

Symptoms can quickly deteriorate from a headache, blurred vision and faintness to complete loss of control over movement and seizures. 

John Hopkins University estimates it affects five in a million people in the US every year, which would suggest 330 patients in Britain suffer from the condition annually.

According to the university, it can affect patients with low blood pressure, cancer, vascular diseases and those prone to blood clotting. Head injuries can also trigger the condition.

Britain’s regulator said CSVT is so rare they aren’t even sure how common it is in the general population.

CVST is a condition in which a clot develops in a vein leading out of the brain, which can cause a stroke.

Drug regulators are particularly worried about this condition developing alongside a reduction in blood platelets, but the researchers didn’t have detailed enough data to study this specifically. 

Professor Paul Harrison, a psychiatrist and one of the researchers on the study, said: ‘We’ve reached two important conclusions. Firstly, Covid 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 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.’

Britain’s medicines chiefs have decided to offer the AstraZeneca jab to under-30s any more because they weren’t sure that their Covid risk was significantly higher than their risk from the vaccine. 

But this study found that younger age groups were still far worse affected after Covid. 

In total 30 per cent of the CVST cases were found in people younger than 30.

This suggests that even if vaccines do increase the risk of CVST in younger people – as experts suspect – Covid increases it more.

Dr Max Taquet said in a briefing: ‘My best guess is that age is not going to have an impact. 

‘CVT occurs across the age range. There’s not more cases of CVT in younger patients.’ 

The team said they had sent the data to Professor Chris Whitty, England’s chief medical officer, and the Joint Committee on Vaccination and Immunisation, but added: ‘It’s not our job to make policy or change it’.

On whether the potential risk of getting a vaccine was worth it for people living in areas with low Covid rates, the researchers insisted jabs were the top priority.

Dr Taquet said: ‘You might live in an area with no Covid cases, so you might think you don’t need the vaccine, but to keep it at zero you need people to take the vaccine.’

Professor Harrison added: ‘All the evidence we have is that the risks of Covid are very much greater than anything [linked to the vaccine].

‘You might think you’re not going to catch it but anyone can get it.’

The researchers said their findings should be interpreted with caution because they come from different sources.

The study is published online but has not yet been reviewed by a journal. 

It comes as another Oxford professor, Sir John Bell, said on Sky News: ‘The best way, if you want to have a bad clotting problem, is to get Covid.

‘And if you don’t get a vaccine you’re going to get Covid, and if you get Covid you’ll have a very, very much higher risk of getting a bad clotting problem.

‘So the clotting problems of the vaccine are pretty trivial compared to the real risks of getting clotting problems if you get Covid.’

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