A coronavirus infection presents a much higher risk of developing a blood clot than the first dose of either the Oxford/AstraZeneca or the Pfizer/BioNTech jab, a large study led by the University of Oxford said on Friday.

The research used findings from more than 29 million people who were vaccinated with the first doses of either vaccine between December 2020 and April 2021.

The findings showed although there was an increased risk of having a blood clot after having the first doses of either vaccine, it was much greater in someone who had tested positive for COVID-19 caused by the SARS-CoV-2 virus, reported news agency Press Trust of India.

"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," said Julia Hippisley-Cox, Professor of Clinical Epidemiology and General Practice at the University of Oxford and lead author of the paper.

The study covered thrombocytopenia - a condition with low platelet counts - and thromboembolic events (blood clots) following vaccination for COVID-19, some of the same events which had led to restricted use in a number of countries of the Oxford/AstraZeneca vaccine - being produced and administered in India as Covishield.

Writing in the 'British Medical Journal'' (BMJ), the researchers detail the findings from over 29 million people vaccinated with first doses of either the ChAdOx1 nCov-19 or Oxford/AstraZeneca vaccine or the BNT162b2 mRNA or Pfizer/BioNTech vaccine.

They conclude that with both of these vaccines, for short time intervals following the first dose, there are increased risks of some haematological and vascular adverse events leading to hospitalisation or death.

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 SARS-CoV-2 coronavirus than after either vaccine.

"This research is important as many other studies, while useful, have been limited by small numbers and potential biases. Electronic healthcare records, which contain the 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," explains Prof Hippisley-Cox.

All of the coronavirus vaccines currently in use have been tested in randomised 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.

Aziz Sheikh, Professor of Primary Care Research & Development and Director of the Usher Institute at the University of Edinburgh and a co-author of the paper, 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 the 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," Sheikh said.

In the paper, the team of authors from the University of Oxford, University of Leicester, Guys and St Thomas'' NHS Foundation Trust, the Intensive Care National Audit & Research Centre, the London School of Hygiene and Tropical Medicine, the University of Cambridge, the University of Edinburgh and the University of Nottingham, compared rates of adverse events after vaccination with Pfizer/BioNTech and Oxford/AstraZeneca vaccines with rates of the same events after a positive SARS-CoV-2 test result.

For this, they 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), using data collected from across England between December 1, 2020 and April 24, 2021.
Other outcomes studied were cerebral venous sinus thrombosis (CVST), ischemic stroke, myocardial infarction and other rare arterial thrombotic events.