The risk of blood clots in the brain due to COVID-19: disease and vaccination. Comments that include information from a non-peer-reviewed study that has not yet been published


According to research from the University of Oxford, COVID-19 disease is associated with a much higher risk of
cerebral venous thrombosis than the risk from the COVID-19 vaccination.

Researchers used electronic records from large U.S. databases to compare the incidence of cerebral venous
thrombosis (CVT) in patients two weeks after being diagnosed with COVID-19, with the incidence of CVT in patients
two weeks after receiving the vaccine. In 513,284 patients diagnosed with COVID-19, the incidence of CVT was
39.0 per million people (confidence interval 95%: 25.2 to 60.2 per million). In the 489,871 patients receiving the
vaccine, the incidence was 4.1 per million (95% CI: 1.1 to 14.9 per million), for an adjusted relative risk of 6.36,
with P<0.001. The patients had been vaccinated with mRNA-based vaccines – Pfizer-BioNTech or Moderna. Patients
were not paired by age or sex. The authors reported that the data did not indicate an obvious association between
age and sex, and occurrences of cerebral venous thrombosis. The comparison was made with mRNA vaccines, not
with viral vector vaccines.

The European Medicines Agency (EMA) has estimated the risk of CVT following administration of the AstraZeneca
vaccine at 5.0 per million people (95% CI: 4.3 to 5.8 per million). Thrombocytopenia associated with an immune
response has been considered to be associated with occurrences of CVT following vaccination with the AstraZeneca
vaccine, specifically with antibodies to platelet factor 4, which causes blood clotting and consumes platelets. These
antibodies have been identified in patients who have experienced blood clots. Thirty percent of cases of CVT due to
COVID-19 occurred in patients under the age of 30.


Torjesen I. Covid-19: Risk of cerebral blood clots from disease is 10 times that from vaccination, study finds. BMJ 2021;
373:n1005 doi:10.1136/bmj.n1005

Taquet M, Husain M, Geddes JR, Luciano S, Harrison PJ. Cerebral venous thrombosis: a retrospective cohort study of 513,284
confirmed COVID-19 cases and a comparison with 489,871 people receiving a COVID-19 mRNA vaccine. OSF. doi
10.17605/OSF. IO/H2MT7

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