The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has launched a probe into a potential association with the jab.
Parents have been told to be in touch with the site where their child received the vaccine if they have concerns.
But the University of Oxford made clear that no problems had arisen within the trial itself.
It said, however, that it was waiting for the results of the MHRA review before it administered any more doses to children.
An Oxford University spokesperson said in a statement: “Whilst there are no safety concerns in the paediatric clinical trial, we await additional information from the MHRA on its review of rare cases of thrombosis/thrombocytopaenia that have been reported in adults, before giving any further vaccinations in the trial.”
They added that parents and children should continue to attend all scheduled visits “and can contact the trial sites if they have any questions".
Earlier, Boris Johnson defended the AstraZeneca vaccine during a visit to the pharmaceutical giant’s manufacturing plant in Macclesfield.
The prime minister praised the work of the MHRA, but said that when it came to the vaccine, the advice was “to keep going out there, get your jab, get your second jab”.
Speaking to The Independent, Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, said the UK could “afford to be cautious” in pausing a trial of the Oxford-AstraZeneca vaccine in children, because it has access to other jabs.
“The whole story behind the increased blood clotting is quite opaque. And I still think they don’t really have a good idea about how common this is in the general population,” he said.
Dr Clarke said that if a link is found between a certain age group and blood clots, then “there are other options … It’s not like it’s the only vaccine we’ve got.”
Earlier, a senior official at the European Medicines Agency (EMA) said he believed that there was a link between the AstraZeneca vaccine and extremely rare cases of blood clots reported in people who had recently had the jab.
In an interview with Italy’s Il Messaggero newspaper, Marco Cavaleri, head of vaccines strategy at the EMA, said it was “clear there is a link with the vaccine” but that there was still uncertainty about what exactly was causing such a reaction.
Mr Cavaleri said that among younger vaccinated people there was a higher than expected number of cases of cerebral thrombosis – blood clotting in the brain – compared with the general population.
A spokesperson for the EMA told The Independent that the Pharmacovigilance Risk Assessment Committee, which is reviewing the data, had “not yet reached a conclusion and the review is currently ongoing”, with an announcement expected on Wednesday or Thursday.
Europe’s regulator insisted as recently as last week that the “causal link with the vaccine is not proven” and continues to recommend that people take the opportunity to get vaccinated when it is offered.
Some countries, including Germany, France and Canada, have paused or restricted use of the vaccine to certain groups.
The World Health Organisation has said the benefits outweigh the risks, while AstraZeneca said in March that its vaccine was found in a US trial to be 76 per cent effective in preventing symptomatic infections, and that studies did not indicate higher risks of clotting.
Mr Cavaleri said the EMA’s evaluation of the AstraZeneca vaccine was “far from over” and that it was for individual countries to establish specific guidelines around which age groups were given which vaccines.
But he added: “In my opinion, we can now say it is clear there is a link with the vaccine. What causes this reaction, however, we still do not know.”
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