Researchers say complications are higher still if someone's infected, meaning jabs still outweigh the risk.
Researchers from the University of Oxford have today announced the results of a study into blood clots following Covid-19 jabs, which led to restricted use of the Oxford-AstraZeneca vaccine in a number of countries.
Writing in the British Medical Journal (BMJ), they detail the findings from over 29 million people vaccinated with first doses of either the Oxford or Pfizer vaccine.
Their findings suggest the risk of thrombocytopenia – a condition where the patient has a low count of cells, known as platelets, that help the blood clot – in someone with coronavirus is almost nine times higher than in someone who has had one dose of the AstraZeneca jab.
Professor Aziz Sheikh, one of those involved in the study, said the increased risk of thrombocytopenia seen in their work is similar to other commonly used vaccines in the UK, such as the flu jab.
With both of the vaccines, for a short time following the first dose, the study found there are increased risks of some haematological and vascular adverse events leading to hospitalisation or death.
The scientists also found an association between those vaccinated with the Pfizer jab and an increased risk of stroke, but the risk was more than 10 times greater in those with the virus.
The authors further note that the risk of these adverse events is substantially higher and for a longer period of time, following infection from the Coronavirus, than after either vaccine.
NEW: An Oxford study has found that the risk of blood clots is far higher following infection from COVID-19 than after taking a vaccine dose.— University of Oxford (@UniofOxford) August 27, 2021
Researchers analysed data from over 29 million vaccinated people.
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The paper, which is the biggest study of its kind on this issue, comes after a coroner concluded that an award-winning BBC radio presenter died due to complications of the Oxford/AstraZeneca vaccination.
Lisa Shaw died in May, just over three weeks after she had her first dose.
The coroner said she had developed a vaccine-induced thrombosis and thrombocytopenia, which was described as “a rare and aggressive complication associated with the AstraZeneca vaccine, which was the underlying cause of her death”.
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 the virus."
All of the Coronavirus vaccines currently in use have been tested in randomized clinical trials, which are unlikely to be large enough to detect very rare adverse events.
When rare events are uncovered, then regulators perform a risk-benefit analysis of the medicine; to compare the risks of the adverse events if vaccinated versus the benefits of avoidance of the disease - in this case, Covid-19.
Prof. Hippisley-Cox 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."