A top official in Europe's medicines regulator said on Tuesday there was a link between AstraZeneca's Covid-19 vaccine and rare blood clots.
The European Medicines Agency's head of vaccines Marco Cavaleri said the body would announce a link between blood clots and the shot later on Tuesday.
"In my opinion, we can say it now: it is clear there is a link with the vaccine. But we still do not know what causes this reaction," he told Italy's Il Messaggero newspaper.
"In the next few hours, we will say that there is a connection, but we still have to understand how this happens.”
In response to Mr Cavaleri's comments, the Amsterdam-based EMA said in a statement on Tuesday: "EMA’s Pharmacovigilance Risk Assessment Committee (PRAC) has not yet reached a conclusion and the review (of any possible link) is currently ongoing."
The EMA said last week that its review had at present not identified any specific risk factors, such as age, gender or a previous medical history of clotting disorders, for the rare events.
A high proportion among the reported cases affected young and middle-aged women but that did not lead the regulator to conclude this cohort was particularly at risk from AstraZeneca's shot.
Scientists are exploring several possibilities that might explain the extremely rare brain blood clots that occurred in individuals in the days and weeks after receiving the vaccine.
One theory suggests that the vaccine triggers an unusual antibody in some rare cases, while other investigators are looking into a possible link with birth control pills.
The agency said last month the vaccine was safe and effective but it was unable to definitively rule out the link between thrombosis and the drug.
Several European nations restricted the vaccine to older people. Others suspended its use completely.
UK Vaccines Minister Nadhim Zahawi said on Tuesday the government would follow the advice of the British medicines regulator after reports officials were preparing to limit use of the vaccine to people over 30.
“We’ve done over 20 million vaccinations using the Oxford-AstraZeneca vaccine ... at the moment, they’ve been very clear to say, when you get your invite come forward and get the vaccine," he told Sky News on Tuesday.
"But we will be led by what the regulator and scientists say is the right thing to do. We stand ready ... to implement whatever they decide."
Questions on whether rare but serious blood clots among those receiving the AstraZeneca injection are more frequent than in the general population, and what causes them if they are, continue to undermine confidence in the vaccine.
After several countries suspended their use of the shot, the EMA said the benefits outweigh the risks and it should remain in use.
But it has said that a causal link between clots and the vaccine is possible. It is expected to provide an updated assessment this week.
"We are trying to get a precise picture of what is happening, to define in detail this syndrome due to the vaccine," Mr Cavaleri said.
"Among the vaccinated, there are more cases of cerebral thrombosis ... among young people than we would expect."
Which countries have restricted AstraZeneca's Covid-19 vaccine?
UK Available for over-18s
France Available for over-55s only
Germany Available for over-60s in most cases
Italy Available for over-65s
Spain Available for under-65s in most cases
Sweden Available for use in over-65s
Norway Vaccine suspended
Netherlands Vaccine suspended