A study by the Oxford University released on Thursday found that the vaccine it jointly developed with AstraZeneca increases the risk of developing blood clots.
However, the university added that it is still much safer to get the vaccine than contracting COVID-19.
The study looked into the prevalence of CVT – cerebral venous thrombosis. In those who took the AstraZeneca vaccine, the CVT occurred in five in a million people. For those who contracted COVID-19, the rate was 39 in a million people.
They studied 500,000 coronavirus patients and found the risk of CVT was approximately 100 times higher than normal.
The MHRA, the UK’s independent medicines regulatory agency, has ruled that the AstraZeneca vaccine is still safe and effective but advised that those under 30 be given an alternative vaccine as their risk from severe COVID-19 is lower.
The Oxford study found that 30% of CVT reports were in people under 30.
The vaccine has run into significant problems in the EU, however, having been restricted to certain age groups or, in Denmark’s case, been dropped from the vaccination program altogether due to blood clotting concerns.
There is a growing scientific consensus that the issue with the AstraZeneca vaccine is its delivery mechanism, known as an adenovirus.
Johnson & Johnson also use this delivery mechanism, and this explains why they too have had issues with regards to blood clots. The US has paused its rollout of the J&J vaccine, and in the EU it has been delayed.
The other major vaccines, Pfizer-BioNtech and Moderna, use a different delivery system known as mRNA technology.
The Oxford study found that the rate of CVT with these vaccines was just four in a million people, and with these two vaccines, people are still around 10 times less likely to develop the CVT than if they contracted COVID-19.
Professor Paul Harrison, from the University of Oxford, said: “We've reached two important conclusions. Firstly, COVID-19 markedly increases the risk of CVT, adding to the list of blood clotting problems this infection causes.
“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.”
Speaking to Sky News, Professor Sir John Bell, Oxford University's regius professor of medicine, said: “The best way, if you want to have a bad clotting problem, is to get COVID.
“And if you don't get a vaccine you're going to get COVID, and if you get COVID you'll have a very, very much higher risk of getting a bad clotting problem.
“So, the clotting problems of the vaccine are pretty trivial compared to the real risks of getting clotting problems if you get COVID.”