Blood clots should be listed as a "very rare side-effect" of the AstraZeneca vaccine but the benefits of preventing Covid-19 still outweigh the risks of getting the shot, the EU's medicines regulator said on Wednesday.
Safety experts said they had found a possible link after studying 86 reports of unusual blood clots among 25 million people who had taken the vaccine in Europe, including the UK.
Several countries, including Britain, have halted the use of the vaccine for younger people over the blood clot scares, which have mainly occurred in women under 60.
But the European Medicines Agency, which in March declared the vaccine to be safe and effective, said in its closely watched verdict that it could not confirm whether there were "specific risk factors" for blood clots such as age or gender.
"Based on the current available evidence, specific risk factors such as age, gender or previous medical history of clotting disorders have not been able to be confirmed,” said EMA director Emer Cooke.
"Our safety committee has confirmed that the benefits of the AstraZeneca vaccine in preventing Covid-19 overall outweigh the risks of side-effects," Ms Cooke said.
"It is very important that we use the vaccines we have to try and beat this pandemic."
EU safety experts said they had studied 62 cases of cerebral venous sinus thrombosis and 24 cases of splanchnic vein thrombosis, leading to a total of 18 deaths.
The EMA said a "plausible explanation" for the blood clots was an immune response triggered by the vaccine.
In the UK, the Joint Committee on Vaccination and Immunisation (JCVI) said that those under 30 should be given Pfizer or Moderna vaccines instead where possible.
"We are advising a preference for one vaccine over another vaccine for a particular age group, really out of the utmost caution, rather than because we have any serious safety concerns," said Wei Shen Lim, the JCVI's Covid-19 chairman.
The success of the UK's vaccine programme is crucial to Prime Minister Boris Johnson’s ambition to lift all legal restrictions on social contact on June 21.
The UK has ordered 100 million doses of the AstraZeneca vaccine, and 40 million doses of Pfizer-BioNTech.
The rulings on Wednesday come after a senior EMA official said this week that there was a "clear" connection between the AstraZeneca-Oxford vaccine and clots.
Canada, France, Germany and the Netherlands are among countries that are not recommending the shot for younger people, although the World Health Organisation also insists the benefits of the jab largely outweigh the risks.
AstraZeneca has said previously that its studies have found no higher risk of blood clots in those vaccinated than in the general population.
The controversy has marred a global vaccination programme that governments hope will help countries emerge from a pandemic that has ravaged economies and subjected much of humanity to some form of confinement.
Dr Peter English, who formerly chaired the British Medical Association’s Public Health Medicine Committee, said the back-and-forth over the AstraZeneca vaccine globally could have serious consequences.
“We can’t afford not to use this vaccine if we are going to end the pandemic,” he said.
The ruling on the vaccine comes as countries from Germany to Ukraine and India face new waves of infections and deaths from the virus that has now killed more than 2.8 million globally.
Governments are scrambling to secure much-needed vaccine doses, with Australia the latest nation to complain of shortages that it blamed on EU export controls.