The spokesperson said the trial in children had not raised any safety concerns, but it would be paused while Britain's Medicines and Healthcare products Regulatory Agency, or MHRA, reviews rare cases of people who suffer blood clots while at the same time having low levels of blood platelets.

"Parents and children should continue to attend all scheduled visits and can contact the trial sites if they have any questions," the spokesperson said.

When contacted by CNN, AstraZeneca referred queries to Oxford University.

Last week, the medicines regulator said at least 30 people in the country had experienced rare types of blood clots after receiving the vaccine, but warned it was too early to know whether the shot itself triggered the clots.

The head of MHRA, Dr. June Raine, told CNN on Tuesday that the MHRA was "aware of the decision taken by the University of Oxford to pause dosing in the trial...whilst the MHRA safety review is ongoing."

"Participant safety in any clinical trial is our top priority, and no safety concerns have been reported with this trial," she added.

MHRA said in a statement it received 22 reports of cerebral venous sinus thrombosis (CVST), where clotting forms in the sinuses that drain blood from the brain, and eight other reports of thrombosis among a total of 15.8 million people who had been given at least one dose of the vaccine by March 21.

They did not say how many blood clots might otherwise have been expected among 15.8 million people

The agency has advised the UK to continue administering the vaccine in all groups, arguing that such clotting incidents are very rare and that the benefits still greatly outweigh the risks, echoing similar assessments by the European Medicines Agency (EMA) and the World Health Organization (WHO).

Regulators in other countries have also reported blood clots among people who have received the vaccine, particularly in Europe, where the AstraZeneca vaccine is widely used. Some countries are choosing to suspend the vaccine altogether, while others have limited its use to certain age groups.

Blood clots in general are so common that it is expected that a certain number of people will get them for various reasons at any given day of any given week. If someone has had a vaccine and then develops a blood clot, it doesn't necessarily mean the shot caused the clot.

After initial reports of clotting last month, AstraZeneca was quick to point out that the incidence of clots overall is lower in people who have received the shot that in the general population.

An AstraZeneca spokesperson said in a statement to CNN last week: "Patient safety remains the Company's highest priority," and pointed to authorities in the UK, EU and the WHO's recommendations to continue its use.

"The benefit risk profile of the vaccine was reaffirmed in the EMA's monthly safety update," the spokesperson said.