The risk of blood clotting diseases caused by the COVID-19 vaccines is significantly outweighed by the risk of being infected with the virus and contracting those same diseases, British researchers say today.
The Oxford University-led study, published in The BMJ, provides a direct comparison for the first time of the risks of haematological and vascular illnesses from vaccination and non-vaccination.
It also shows how the vaccines may be triggering a very small number of cases of illnesses commonly associated with COVID-19 infection.
Researchers compared 29 million people after their first dose with one of the UK’s two main vaccines – one adenoviral, one mRNA – with 1.76 million people who had a positive test for infection with the virus.
Eight research institutions collaborated in the project, including six universities.
They found that several hundred people have suffered serious illness or death after vaccination from thrombocytopaenia, venous thromboembolism (VTE)and ischaemic stroke. However, they found thousands of cases of the same illnesses among people who tested positive for infection.
Most significantly, cases of VTE occurred at a rate of 12,614 per ten million amongst those who were infected – compared with 66 per ten million amongst those who received the AstraZeneca adenoviral vaccine – ChAdOx1nCoV-19.
Stroke risk increased
The risk of ischaemic stroke was about ten times greater after infection than after vaccination.
Lead researcher Professor Julia Hippisley-Cox, a professor of general practice, 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.”
She added: “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.”
Professor Aziz Sheikh, from the University of Edinburgh, said: “This enormous study, using data on over 29 million vaccinated people, has shown that there is a very small risk of clotting and other blood disorders following first dose COVID-19 vaccination. Though serious, the risk of these same outcomes is much higher following SARS-CoV-2 infection.
“On balance, this analysis therefore clearly underscores the importance of getting vaccinated to reduce the risk of these clotting and bleeding outcomes in individuals, and because of the substantial public health benefit that COVID-19 vaccinations offer.”
The UK yesterday reported 38,281 new cases of infection and 140 new deaths from the virus. In total 6.6 million people in the UK have tested positive for infection, a rate of 97,058 cases per million population.
Risk of thrombocytopenia and thromboembolism after COVID-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study. BMJ 27 August 2021
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