A British trial of the AstraZeneca coronavirus vaccine on children has been paused while regulators assess its possible link to blood clots, said Oxford University, which helped develop the jab.
"Whilst there are no safety concerns in the paediatric clinical trial, we await additional information from the MHRA [UK regulator] on its review of rare cases of thrombosis/thrombocytopaenia that have been reported in adults, before giving any further vaccinations in the trial," the university said in a statement.
"Parents and children should continue to attend all scheduled visits and can contact the trial sites if they have any questions."
Britain's Medicines and Healthcare products Regulatory Agency (MHRA) is one of many bodies across the globe analysing real world data from the AstraZeneca rollout to see if there is a definitive link between the jab and a rare form of blood clot, after cases were initially reported in Norway and continental Europe.
The World Health Organization and the European Medicines Agency (EMA) will reveal their findings later this week.
It is the latest setback for AstraZeneca, which has been embroiled in controversy over its failure to deliver its promised doses to the European Union, and over the jab's efficacy and safety profile.
The MHRA reported over the weekend that there had been 30 blood clotting cases, seven fatal, out of the 18 million doses administered in Britain.
The EMA said it "has not yet reached a conclusion and the review is currently ongoing".
European Union Health Commissioner Stella Kyriakides later said that the agency was expected to make its decision "late Wednesday", adding that she was in "close contact" with the EMA.
The statement came after the EMA's head of vaccine strategy Marco Cavaleri was quoted in Italian media as saying that there was a "clear" connection and that the agency would announce it within hours.
"In my opinion, we can say it now, it is clear there is a link with the vaccine," Cavaleri told Italy's Il Messaggero newspaper in an interview. "But we still do not know what causes this reaction."
In Britain, Sage (Scientific Advisory Group for Emergencies) adviser Professor Calum Semple urged people to continue accepting Oxford/AstraZeneca jabs, telling Channel 4 News: "This has been done out of exceptional caution and the big story still is that for a middle-aged, slightly overweight man, such as myself, my risk of death is one in 13,000 - the risk of this rare clot, which might not even be associated with the vaccine, is probably one in a million.
"So I'm still going to say it's better to get the vaccine than not get the vaccine and we can pause and take time to carefully consider the value for children because they're not at risk of death from Covid."
He added: "If you've been called for the vaccine then you're in an age group that is very likely to benefit from the vaccine. So the bottom line is if you've been called for the vaccine I would urge you to take the vaccine."