The finding, which came from more than 29 million people, showed there is still a risk after having the jab – but it is “considerably higher” in someone who has the virus. The researchers, from the University of Oxford, are fully independent of the team that worked on the AstraZeneca jab. It comes after heightened concerns earlier in the year led to the restricted use of the AstraZeneca vaccine in a number of countries.
The new research, published in the British Medical Journal, found for short time intervals after the first dose there were rare cases of hospitalisation or death from blood clots.
But, importantly, they also found the risk of these adverse events is substantially higher and for a longer period of time, following infection from coronavirus.
Julia Hippisley-Cox, 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 the vaccines currently on offer to fight COVID-19 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.
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 December 1, 2020, and April 24, 2021.
Professor 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.”
READ MORE: Coronavirus breakthrough as AstraZeneca hails results of vaccine alternative treatment
The new data comes after AstraZeneca hailed the results of vaccine alternative treatment.
The antibody therapy reduced the risk of participants developing COVID-19 symptoms by 77 percent, according to the drugmaker.
It opens up the door to an alternative option for people who may not see the full protective benefits from the currently authorised vaccines.
The PROVENT pre-exposure prophylaxis trial showed AZD7442 could potentially provide long-lasting protection with demonstrated clinical trial success.
AstraZeneca said that 75 percent of participants in the stage three trial — comprising two types of antibodies that were discovered by Vanderbilt University Medical Centre in Tennessee — had chronic conditions, including some with a lower immune response to vaccinations.