COVID-19 – the actual disease – poses 8 to 10 times the threat of blood clots in the brain, as coronavirus vaccines do, finds a large, non-peer-reviewed study led by researchers at the University of Oxford.
The study, published today on preprint server OSF, involved an electronic health record network of 81 million patients in 59 healthcare systems, primarily in the United States.
The researchers counted patients diagnosed with cerebral venous thrombosis (CVT) in the two weeks after receiving their COVID-19 diagnosis or their first dose of Pfizer / BioNTech or Moderna vaccine from 20 January 2020 to 25 March 2021. The researchers compared those with the number of CVTs in an unmatched group who had the flu during the same period and in the general population.
Only 2 cases in vaccinated
The risk of CVT was 8 to 10 times higher in the 513,284 patients with a COVID-19 diagnosis than in the 489,871 vaccinations and 100 times greater than in the general population. There were only two cases of CVT in the vaccine population, one after vaccination with the Pfizer vaccine and one after receiving an indefinite mRNA vaccine. Patients younger than 30 years accounted for 30% of coronavirus-related CVT. The percentage of the 172,742 flu patients was 0 per 1 million.
CVT in all patient groups was rare, with 39 per 1 million COVID-19 patients and 4 out of 1 million vaccine recipients. The risk of CVT after COVID-19 was about 10 times higher than that of a single dose of Pfizer or Moderna vaccines, and according to the European Medicines Agency (EMA) about eight times higher than after the AstraZeneca / Oxford vaccine. (The researchers could not determine this in the study population because this vaccine is not used in the United States.)
The mortality rate among COVID-19 patients was 20% for those who had CVT and 18.8% for portal vein (liver) thrombosis (PVT), which were assessed in the same populations.
The prevalence of PVT was 436.4 per 1 million people with COVID-19, 98.4 per 1 million among influenza patients and 44.9 per 1 million after vaccination with the Pfizer or Moderna vaccine. Twenty-two cases of PVT were diagnosed under vaccines, 11 after the Pfizer vaccine, 2 after the Moderna vaccine and 9 after a vaccine of an indefinite brand (Pfizer or Moderna).
Warnings and context
Recent reports on blood clots after receiving the AstraZeneca and Johnson & Johnson COVID-19 vaccines in young women led to a halt in the rollout of the two adenovirus vaccines in Europe (AstraZeneca) and the United States (Johnson & Johnson ).
For example, yesterday, the New England Journal of Medicine published a letter to the editor of Doctors of the University of Nebraska Medical Center in Omaha outlining the case of an otherwise healthy 48-year-old woman who became critically ill with thrombosis in the veins or the formation of blood clots in unusual places in the body, 2 weeks after receiving the Johnson & Johnson vaccine.
Study co-author Paul Harrison, MD, said in the Oxford University news release that their study shows the dramatic increase in the risk of CVT due to COVID-19. “The COVID-19 risk is higher than what we see with current vaccines, even for those under 30; something to consider when considering the balance between risks and vaccine benefits,” he said.
In the release, lead author Maxime Taquet, PhD, said the study data should be interpreted with caution because the European Medicines Agency’s AstraZeneca vaccine data were monitored, while data on the other vaccines came from the TriNetX electronic health record network. “However, the signals that COVID-19 is linked to CVT, as well as portal arterial thrombosis – a coagulation disorder in the liver – are clear, and we need to take note,” he said.
David Werring, PhD, at University College London, said among the expert responses today on the Science Media Center website that the study has methodological limitations. ‘In particular, the authors were unable to verify the accuracy of the diagnosis of CVST [cerebral venous sinus thrombosis], which we know can be challenging, requiring appropriate clinical expertise and timely scanning of the arteries in the brain, ”he said.
“Furthermore, they were unable to assess the risks of CVST associated with the AstraZeneca vaccine in the same population.”
Kevin McConway, PhD, of Open University, UK, said the results should reassure the public that the risk of CVT due to vaccinations is low.
“The researchers do not claim that vaccines do not increase the risk at all compared to the risk in people who have not been vaccinated and have not had COVID-19 – but they do say that the CVT risk in people who have had COVID-19 “is about 100 times the risk in the general population,” he said. “I do think it puts things in context.”
The authors asked for future research on whether coronavirus-related CVT occurs under the same or a different mechanism than in the vaccine-related complication.