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blood clots and COVID
by Rod Tucker
Published on 20 April 2021

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Blood clots more likely from COVID-19 than vaccines

With much concern over the risk of blood clots from COVID-19 vaccines, research suggests that clots are far more likely to occur through infection with the virus.

International concerns over the risk of blood clots from vaccination with the Oxford-AstraZeneca COVID-19 vaccine, ChAdOx1 have led to some countries suspending its use. The risks have been centred on a cerebral venous thrombosis (CVT), which occurs when a blood clot forms in the brain’s venous sinuses, preventing blood from draining out of the brain. It is a rare form of stroke that affects about 5 people in a million each year. Exactly how the vaccine causes a CVT remains to be established but one suggestion is that it arises because of what has been described as a “vaccine-induced thrombotic thrombocytopenia (VITT). However, one factor which has received little attention is the extent to which infection with COVID-19 itself causes CVT. This led a team from Oxford university, to estimate the incidence of CVT in the two weeks following a COVID-19 diagnosis. The researchers turned to the electronic health records network to examine the incidence of CVT in those with COVID-19 and compared this to two other groups: those who received COVID-19 vaccines and a group who had been diagnosed with influenza.

Findings
A total of 513,284 patients with a confirmed diagnosis of COVID-19 with a mean age of 46.6 years (54.8% female) were included in the study. Of these, 20 were diagnosed with CVT in the two weeks following their COVID-19 diagnosis, giving an absolute risk of 39 per million people. Moreover, the risk was considerably higher among those with a history of cardiovascular diseases. There appeared to be no link with influenza infection (0 cases per million people) but the risk from COVID-19 was considerably higher than the risk from vaccination (4.1 per million people). When looking at a broader definition of CVT in terms of the coding used, the incidence in the 2-weeks following COVID-19 was 171.4 per million people, compared to 52.1/million for influenza and 22.7 per million among those vaccinated.

In discussing their findings, the authors estimated that the risk of developing a CVT was 8-times higher in patients with COVID-19 compared with those who have been vaccinated. While recognising the need larger sample sizes to confirm their findings, the authors concluded that the risk of CVT is much higher from COVID-19 than from vaccination and that their results should help inform the debate over the risk-benefit ratio for vaccines.

Citation
Taquet M et al. 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.



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