Highlights:
- Rare blood clotting risk due to COVID-19 infection is several times higher than post-vaccination.
- According to the study, the risk is approximately 8-10 times higher.
- The findings come as a number of countries have restricted or stopped using the Oxford/AstraZeneca vaccine in certain age groups.
According to a new University of Oxford study published on Thursday, the risk of rare blood clotting after COVID-19 infection is about 100 times higher than average, many times higher than it is after vaccination or influenza.
The study discovered that Cerebral Venous Thrombosis (CVT), a rare blood clotting condition, is more common after COVID-19 than in any of the comparison groups, with 30% of cases occurring in people under the age of 30.
This risk is about 8-10 times higher than the current COVID-19 vaccines in use, and about 100 times higher than the baseline.
The research comes after reports of possible link between the Oxford/AstraZeneca vaccine and unusual cases of CVT, which are being investigated further by medicines regulators despite the fact that the vaccines have been deemed safe and reliable.
Prof. Paul Harrison, who is also one of the study’s authors and Head of the Translational Neurobiology Group, University of Oxford, said, “Because of concerns regarding potential links between vaccines and Cerebral Venous Thrombosis (CVT), governments and regulators have imposed restrictions on the use of certain vaccines. However, one central question remained unanswered: ‘What is the chance of CVT after a COVID-19 diagnosis?”
“We’ve come to two main conclusions. To begin with, COVID-19 significantly raises the risk of CVT, contributing to the long list of blood clotting issues that this infection triggers.
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“Secondly, even for those under 30, the COVID-19 risk is higher than we see with current vaccines,” he said, adding that this should be taken into account when weighing the pros and cons of vaccination.
Professor Harrison and Dr. Maxime Taquet of Oxford University’s Department of Psychiatry and the NIHR Oxford Health Biomedical Research Centre led a team of researchers who counted the number of CVT cases diagnosed in the two weeks following a COVID-19 diagnosis or the first dose of a vaccine. They then applied these to estimated CVT incidences following influenza and the general population’s background level.
Dr Maxime Taquet, also of the Translational Neurobiology Group, said, ” COVID-19 has a close correlation to CVT and portal vein thrombosis, a clotting disease of the liver, and it’s something we should pay attention on it.”
CVT occurred in 39 out of a million patients in a sample of over 500,000 COVID-19 patients. CVT was registered in four out of every million people who received a COVID-19 mRNA vaccine, such as Pfizer/BioNTech or Moderna, and in around five out of every million people who received the Oxford/AstraZeneca COVID-19 vaccine.
“The chance of a CVT from coronavirus is around ten times higher than the risk from COVID-19 mRNA vaccines. The chance of a CVT from COVID-19 is around eight times higher than the AZ-Oxford (Oxford/AstraZeneca) vaccine,” the findings note.
However, the researchers stated that all comparisons should be viewed with caution because data is still being collected. A key question that needs to be investigated further is whether COVID-19 and vaccines cause CVT by the same or different mechanisms. According to the study authors, there may be under-reporting or mis-coding of CVT in medical reports, resulting in confusion about the accuracy of the data.
The results come as a number of countries have restricted or halted the use of the Oxford/AstraZeneca vaccine to specific age groups. Following news of very rare cases of blood clots, Denmark has permanently removed the vaccine from its vaccination programme.
The UK’s Medicines and Healthcare Products Regulatory Agency (MHRA) said earlier this month that the vaccine is still safe and reliable, but that people under 30 will be given an alternative as a precaution.
The benefits of the Oxford/AstraZeneca vaccine tend to outweigh the risks, according to the experts, and the general recommendation, including that of the World Health Organization (WHO), is to continue vaccinations.
