Blood clot risk higher with Covid-19 than vaccination

  • Date: 30 August 2021

VACCINATION for Covid-19 poses much less risk of blood clotting than does the virus itself, according to research published in the BMJ.

The study assessed data from over 29 million people vaccinated with first doses of either the ChAdOx1 nCov-19 ‘Oxford-AstraZeneca’ vaccine or the BNT162b2 mRNA ‘Pfizer-BioNTech’ vaccine. Both vaccines showed increased risks of some haematological and vascular adverse events leading to hospitalisation or death for short time intervals following the first dose – but the risks of these adverse events were much higher following SARS-CoV-2 infection.

Reports of thrombocytopenia and thromboembolic events (blood clots) following vaccination for Covid- 19 have led to restricted use of the Oxford-AstraZeneca vaccine in a number of countries.

The researchers from Oxford University and number of other UK institutions used routinely collected electronic health records to evaluate the short-term risks (within 28 days) of hospital admission with thrombocytopenia, venous thromboembolism (VTE) and arterial thromboembolism (ATE). Other outcomes studied were cerebral venous sinus thrombosis (CVST), ischemic stroke, myocardial infarction and other rare arterial thrombotic events.

The authors concluded that the risk of these adverse events is substantially higher and for a longer period of time following infection from the SARS-CoV-2 ‘coronavirus’ than after either vaccine.

Julia Hippisley-Cox, Professor of Clinical Epidemiology and General Practice at the University of Oxford, lead author of the paper, said: "People should be aware of these increased risks after Covid-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2.

"This research is important as many other studies, while useful, have been limited by small numbers and potential biases. Electronic healthcare records, which contain detailed recording of vaccinations, infections, outcomes and confounders, have provided us with a rich source of data with which to perform a robust evaluation of these vaccines, and compare to risks associated with Covid-19 infection."

Link: Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study

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