Oxford scientists said COVID-19 patients were eight times more likely to develop rare blood clotting from the coronavirus than the AstraZeneca vaccine.
University of Oxford researchers on Thursday reported that the risk of blood clotting following COVID-19 infection was about eight times higher than that of the AstraZeneca vaccine, according to a statement.
The Oxford-developed vaccine has been the center of controversy over reports of rare blood clotting. Several countries have halted its use to investigate the reports.
Coronavirus patients are 100 times more likely to develop blood clotting, known as cerebral venous thrombosis (CVT), than normal, Oxford scientists said.
The study found that about five in a million people developed rare blood clotting after their first dose of the AstraZeneca vaccine.
While four in a million people who received an mRNA jab — BioNTech-Pfizer or Moderna — reportedly also developed CVT.
According to the Oxford study, the risk of CVT following COVID-19 infection was about 10 times higher than that of mRNA vaccines.
"COVID-19 markedly increases the risk of CVT, adding to the list of blood clotting problems this infection causes," said Paul Harrison, professor of psychiatry and head of the Translational Neurobiology Group at the University of Oxford.
"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," Harrison added.
Dr. Maxime Taquet, also of Oxford's Department of Psychiatry, said the data should be interpreted cautiously, since the study was UK-focused.
"However, 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," Taquet added.
The Oxford-developed jab is a so-called "vector vaccine." It uses an adenovirus-vector technology, where the adenovirus is used as a carrier to a modified spike of the virus to trigger an immunity response.
Vaccines that use mRNA technology take a small part of the virus' genetic information to spark an immune response through producing protein directly in the cell.
The blood-clotting reports have prompted concerns over vector vaccines, with similar reports arising around the Johnson & Johnson coronavirus vaccine.
The European Union's drug regulator said the benefits of the AstraZeneca vaccine outweighed its risks after several countries raised concerns over reports of blood clotting.
The European Medicines Agency (EMA) said it had received reports of 169 cases of CVT in people who took AstraZeneca's COVID-19 vaccine, compared to the administered 34 million doses of the shot.
But some countries have cautiously restricted its use, with Germany suspending the use of the AstraZeneca vaccine for people under age 60.
On Wednesday, Denmark announced that it would permanently halt vaccination with the AstraZeneca jab.