A trial of the Oxford/AstraZeneca vaccine on children and teenagers in the UK has been paused while a possible link to blood clots is investigated.
Professor Andrew Pollard from the University of Oxford told the BBC there were no safety concerns with the trial itself, but its scientists were waiting for further information.
In a statement, a spokesperson from the university added: "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.
"Parents and children should continue to attend all scheduled visits and can contact the trial sites if they have any questions."
The trial, which started in February, is assessing whether the jab produces a strong immune response in children aged between 6 and 17.
Around 300 volunteers have signed up.