A coronavirus infection presents a much higher risk of developing a blood clot than a first dose of either the AstraZeneca or the Pfizer jab, a study has found.
The research used findings from more than 29 million people who were vaccinated with first doses of either jab.
The study 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.
The researchers are fully independent from the University of Oxford team that worked with AstraZeneca to develop that particular vaccine.
Fears over blood clots have led to the restricted use of the Oxford AstraZeneca vaccine in a number of countries.
The researchers, writing in the British Medical Journal, found for short time intervals after the first does there are increased risks of some blood, or blood-vessel, related events leading to hospitalisation or death.
However, they also wrote that that the risk of these adverse events is substantially higher and for a longer period of time, following infection from coronavirus.
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.”
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, regulators perform a risk-benefit analysis of the medicine to compare the risk of the adverse reaction, such as a blood clot, against the benefits of avoiding the disease – in this case COVID-19.
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For this paper, the researchers used routinely collected electronic health records to evaluate the short-term risks (within 28 days) of hospital admission from blood clots.
They used data collected from across England between 1 December 2020 and 24 April 2021.
Professor Hippisley-Cox said: “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.”
The research was carried out by a team 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.