A trial of the Oxford-AstraZeneca coronavirus vaccine on children has been paused while the United Kingdom's medicines regulator investigates a possible link with rare blood clots in adults, the BBC is reporting.
Professor Andrew Pollard from the University of Oxford told the BBC there were no safety concerns with the trial itself, but scientists were waiting for further information.
The study began in mid-February and aimed to test the vaccine in more than 200 young people aged six to 17 years.
It is understood there were up to 300 volunteers for the trial.
The paused trial does not affect New Zealand, which is rolling out the Pfizer vaccine.
An Oxford University spokesman stressed that no safety issues have arisen during the study itself - instead the decision is a precautionary measure amid broader concerns about rare blood clots in adults.
The spokesman added that Oxford is waiting for more information from the UK regulator, the Medicines and Healthcare products Regulatory Agency (MHRA), before giving any further vaccinations to children or teenagers.
Channel 4 News reported that the UK's regulator was considering a proposal to restrict the use of the Oxford-AstraZeneca vaccine in younger people.
Earlier, a health chief from the European Medicines Agency (EMA) said there was a link between AstraZeneca's Covid-19 vaccine and blood clots.
"In my opinion, we can say it now, it is clear there is a link with the vaccine," said the EMA's head of vaccines Marco Cavaleri in an interview with Italy's Il Messaggero newspaper.
Cavaleri indicated that the EMA would confirm this view, adding, "in the next few hours, we will say that there is a connection, but we still have to understand how this happens".
He said that "we still do not know what causes this reaction", as reports accumulate of people across the world experiencing blood clots after receiving the jab developed by British-Swedish drug company AstraZeneca and Oxford University.
"We are trying to get a precise picture of what is happening, to define in detail this syndrome due to the vaccine," Cavaleri said.
"Among the vaccinated, there are more cases of cerebral thrombosis … among young people than we would expect."
However, the World Health Organisation said it expected the UN agency will be find no reason to change its assessment that the benefits of the AstraZeneca vaccine against Covid-19 outweigh any risk in an ongoing investigation.
- additional reporting from agencies