Researchers at the University of Oxford say Covid-19 leads to a risk of rare blood clots that's several times higher than that of current vaccines, amid concerns that the jabs may be to blame for the potentially deadly condition.
The report released on Thursday says the risk of the rare condition known as cerebral venous thrombosis (CVT) following Covid-19 infection is “around 100 times greater than normal” and “several times higher” than it is post-vaccination or after flu.
Researchers looked at health records of 500,000 Covid-19 patients to estimate the absolute incidence of CVT in the two weeks following Covid-19 diagnosis, flu, or vaccination against coronavirus. They found that clotting is around eight times more likely to occur after contracting Covid-19 than after immunization with the AZ-Oxford Covid-19 vaccine. Compared to the vaccines made by Pfizer or Moderna, the risk of a CVT from Covid-19 is about 10 times greater.
The researchers, however, call for a cautious interpretation of the comparisons since data is still coming in.
“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?’” says Professor of Psychiatry and Head of the Translational Neurobiology Group at the University of Oxford Paul Harrison.
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The Oxford researchers also examined data on portal vein thrombosis (PVT) – basically, a blood clot in the major vessel that carries blood from the intestines to the liver – and found that the “absolute incidence in the two weeks after Covid-19 diagnosis was 436.4 per million people.” The data showed that the incidence rate of PVT among recipients of an mRNA vaccine such as Pfizer's was almost 30 times higher than those who got the Oxford/AstraZeneca jab, at 44.9 per million people compared to 1.6 per million people respectively.
Some countries have restricted the use of the Oxford-AstraZeneca vaccine – which was recently renamed Vaxzevria – to certain age groups after the European Medicines Agency confirmed the vaccine’s possible links to cases of blood clotting, several of which were fatal. Denmark halted the use of the jab altogether.
Meanwhile, another EMA-approved vaccine, Johnson & Johnson's, is being investigated in the US and Europe following cases of blood clotting and at least one death.
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Both vaccines are based on a similar method, known as adenoviral vectors. Regulators and researchers are working to find out whether the incidences of blood clotting are caused by an immune response to the adenovirus vaccines
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