Researchers at the University of Oxford, who are not linked to the vaccine, also found that people infected with coronavirus are “manyfold times” more likely than normal to develop the rare clotting disorder, known as cerebral venous thrombosis (CVT), where blood clots in the veins that run from the brain.
“There’s no doubt that Covid is a much greater risk of this [condition] than any of the vaccines,” said Professor Paul Harrison, a co-author of the study.
The research, which has yet to be peer-reviewed, drew comparisons between more than 500,000 Covid-19 patients in the US with 34 million people across Europe who have received the AstraZeneca vaccine.
For Covid-19, the incidence rate of CVT stands at 39 cases per one million people, the study showed. But for a million people vaccinated with the AstraZeneca jab, there will be just five cases of CVT over a two-week period.
The scientists warned that all comparisons must be interpreted cautiously since data are still accruing. They added that their research was unable to determine the relative risk of developing CVT after vaccination due to uncertainty around the baseline rate for this condition.
Nor did the study address the incidence rate of thrombocytopenia in Covid-19 patients and people who had been vaccinated. This condition, where a patient presents abnormally low levels of platelets, has been detected alongside CVT in the cases of concern reported to date.
However, Prof Harrison, a professor of psychiatry and head of the Translational Neurobiology Group at Oxford University, said the research highlights “two important conclusions”.
“Firstly, Covid-19 markedly increases the risk of CVT, adding to the list of blood clotting problems this infection causes,” he said.
“Secondly, the Covid-19 risk is higher than seen with the current vaccines, even for those under 30; something that should be taken into account when considering the balances between risks and benefits for vaccination.”
The researchers said that out of the 20 cases of CVT detected among the 500,000 Covid patients, six occurred in people under 30.
“CVT occurs across the age range,” said Dr Maxime Taquet, co-author of the study and a professor of psychiatry at Oxford University. “There’s not an increased rate of CVT plus Covid in the younger patients. There’s just as many young patients as old patients.
“We’ve got preliminary data where we do actually match for age, sex and race, and we see the exact same relative risk.”
The research was described as “opportunistic” by the scientists, as it drew its Covid-19 patient data from a US-based electronic health records network which had recently been used by the Oxford team to show the neurological and psychiatric consequences of coronavirus.
This database, which is made up of a total of 81 million US patients, provided clear detail on CVT cases that had been detected among people infected with coronavirus, the experts said, though they acknowledged there may be under-reporting of the condition in medical records.
As for the incidence rate of CVT among vaccinated people, this was drawn from the European Medicines Agency’s database, which covered more than 34 million individuals at the time of publication.
Professor John Geddes, who was also involved in the research, said the findings of the study would be presented to Professor Chris Whitty, England’s chief medical adviser, and the Joint Committee on Vaccination and Immunisation.
The scientists said their research was not influenced or shaped in any form by the Oxford team who developed the AstraZeneca vaccine.