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 and 34 million people in Europe who have received the AstraZeneca vaccine, as well as the background level of CVT in the general population.
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 the data were drawn from different sources and 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 513,000 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.
The scientists said their research was not influenced or shaped in any form by the Oxford team who developed the AstraZeneca vaccine
“We’ve got preliminary data where we do actually match for age, sex and race, and we see the exact same relative risk.”
The study also suggested that four cases of CVT are likely to occur in one million people vaccinated with either the Moderna or Pfizer/BioNTech jab. However, the scientists warned that the data on this is too thin to establish any firm conclusions.
To determine the rate of CVT in people infected with coronavirus, the Oxford researchers looked at data from a US-based electronic health records network, called TriNetX. This provided detail on CVT cases that had been detected among 513,000 Covid-19 patients, the experts said – though they acknowledged there may be under-reporting of the condition in the records. They were also unable to verify the accuracy of the diagnosis of CVT.
As for the incidence rate of CVT among people vaccinated with the AstraZeneca jab, this was calculated using the European Medicines Agency’s data, which covered more than 34 million individuals who had received a first dose of the vaccine at the time of the study’s publication.
Professor John Geddes, who was also involved in the research, said the findings would be presented to Professor Chris Whitty, England’s chief medical officer, 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.
They also said that research is needed to establish whether Covid-19 and vaccination with the coronavirus jabs lead to CVT by the same or different mechanisms.
Professor Kevin McConway, of the Open University, who was not involved in the study, said: “This new piece of research throws a bit more light, though the researchers make it clear that they do not have all the answers, and that more data and more analysis and interpretation are necessary.”