Researchers at Oxford University have found the risk of a rare type of blood clot is low overall, but higher for people who have been infected with COVID-19 than among people who've had the three vaccines authorised in the UK - those made by AstraZeneca, Moderna and Pfizer.
According to a recent study, the risk of cerebral venous thrombosis or CVT - also known as cerebral venous sinus thrombosis or CVST - following COVID-19 infection is around "100 times greater than normal and several times higher than it is post-vaccination or following influenza," across all age groups.
The WHO on April 15 said "for now the risk of suffering blood clots, is much higher for someone with COVID-19 than for someone who has taken the AstraZeneca vaccine." WHO Regional Director for Europe Hans Kluge reiterated its recommendation of the AstraZeneca vaccine for all eligible adults, calling it "effective in reducing COVID-19 hospitalisation and preventing deaths."
As per an official statement by Oxford University, the study authors, led by Professor Paul Harrison and Dr Maxime Taquet from Oxford University's Department of Psychiatry and the NIHR Oxford Health Biomedical Research Centre, counted the number of CVT cases diagnosed in the two weeks following diagnosis of COVID-19, or after the first dose of a vaccine, reported news agency ANI.
The researchers at Oxford then compared these to calculated incidences of CVT following influenza, and the background level in the general population.
According to them, the CVT is more common after COVID-19 than in any of the comparison groups, with 30 percent of these cases occurring in the under 30s. Besides, the study shows, compared to the current COVID-19 vaccines, this risk is between 8-10 times higher, and compared to the baseline, approximately 100 times higher.
The study found that compared to the mRNA vaccines, the risk of a CVT from COVID-19 is about 10 times greater. Whereas, compared to the AZ-Oxford vaccine, the risk of a CVT from COVID-19 is about 8 times greater.
Paul Harrison, Professor of Psychiatry and Head of the Translational Neurobiology Group at the University of Oxford, said in a statement: "There are concerns about possible associations between vaccines, and CVT, causing governments and regulators to restrict the use of certain vaccines. 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 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," he added.
As per the official statement, an important factor that requires further research is whether COVID-19 and vaccines lead to CVT by the same or different mechanisms. There may also be under-reporting or mis-coding of CVT in medical records, and therefore uncertainty as to the precision of the results.
This development comes after blood clotting cases were reported after people in European Union received doses of the AstraZeneca vaccine since vaccination started.
Recently, Johnson & Johnson's single jab vaccine was halted by the US Food and Drug Administration
authorities following the death of a woman who developed blood clots after receiving the vaccine.