The risk of developing a blood clot with the coronavirus itself is up to 10 times higher than experiencing the complication post-vaccine, research suggests.
Concerns have been raised over the University of Oxford-AstraZeneca jab. As of 31 March, 79 blood clot cases had been linked to the vaccine's first dose, of whom 19 patients died.
"Out of the utmost caution" the UK's jab regulator recommends healthy people under 30 in the UK have one of the other two approved coronavirus vaccines, developed by Pfizer-BioNTech and Moderna.
To better understand the issue, scientists from the University of Oxford – who are not affiliated with the AstraZeneca jab – studied the blood clot risk among coronavirus patients, vaccine recipients and the general population.
Results reveal the risk of developing a cerebral venous thrombosis (CVT) is considerably higher while enduring a severe case of the coronavirus than after one of the three vaccines that protects against it.
A CVT forms in the brain's venous sinuses, preventing blood from draining from the vital organ.
Covid-19, the disease caused by the coronavirus, is thought to make blood stickier and trigger widespread inflammation in extreme cases.
The results are preliminary and yet to be published in a peer-reviewed journal.
"There are concerns about possible associations between vaccines and CVT, causing governments and regulators to restrict the use of certain vaccines," said study author Professor Paul Harrison.
"Yet, one key question remained unknown: 'What is the risk of CVT following a diagnosis of Covid-19?'
"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 [what 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."
The Oxford-AstraZeneca vaccine has specifically been linked to thrombocytopenia; blood clots with low platelets, the cells that promote clotting following a bleed.
An insufficient number of thrombocytopenia cases were in the Oxford team's database to assess this complication specifically, with the scientists instead focusing on the less "nuanced" CVT.
CVT is "at the heart of" the clotting concerns, according to Professor Harrison.
The Pfizer-BioNTech and Moderna jabs are based on the same technology, which differs from the Oxford-AstraZeneca vaccine. The former two jabs have not yet been linked to blood clots.
Using largely US health records, the Oxford scientists counted the number of CVT cases diagnosed in the two weeks after more than half a million people tested positive for the coronavirus.
These were compared against the number of CVT incidences after a first dose of the Pfizer-BioNTech or Moderna vaccines.
The Oxford-AstraZeneca jab is not yet approved in the US, however, the European Medicines Agency (EMA) estimates around five CVT cases occur in every 1 million people after their first dose.
Vaccines aside, the Oxford scientists calculated 39 cases of the clot would be expected to occur for every 1 million people with the coronavirus.
This is compared to zero cases among those with seasonal flu, which was included in the analysis due to it also being a viral respiratory infection.
After assessing more than 480,000 people who received a first dose of the Pfizer-BioNTech or Moderna vaccines, the scientists calculated four in 1 million would develop a CVT after either jab.
This is not dissimilar to the EMA's five per 1 million estimate after the first Oxford-AstraZeneca dose.
The Oxford scientists concluded the risk of developing a CVT is eight times higher while infected with the coronavirus than after the Oxford-AstraZeneca vaccine.
The risk rises to 10 times higher if a person becomes infected rather than having the Pfizer-BioNTech or Moderna jabs, the results suggest.
Coronavirus patients also face around a 100 time higher risk of a CVT than an average person without the infection, the results suggest.
"The incidence of CVT following COVID-19 was higher than the incidence observed across the entire health records network," wrote the scientists.
Perhaps surprisingly, the scientists also found 30% of the CVT cases occurred in coronavirus patients under 30. Coronavirus complications and clots themselves are known to be more common in old age.
The benefits of the Oxford-AstraZeneca vaccine over any risks are said to be more obvious among older people, but become blurred in younger age groups.
This is why the UK's Joint Committee on Vaccination and Immunisation recommends healthy individuals under 30 receive a Pfizer-BioNTech or Moderna jab.
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The Oxford scientists have stressed their study is ongoing and should be interpreted with caution.
Participants were not age matched when comparing the CVT risk among coronavirus patients to those of the general population.
Co-author Dr Maxime Taquet pointed out the data on the Oxford-AstraZeneca jab came from the UK's Medicines and Healthcare products Regulatory Agency (MHRA). Other information was gathered from the electronic health records network TriNetX.
The data's accuracy and "completeness" are also unknown.
Nevertheless, "the signals COVID-19 is linked to CVT is clear, and one we should take note of", added Dr Taquet.
Further research should investigate why the Oxford-AstraZeneca vaccine, and the coronavirus itself, may cause clots, according to the scientists.