LONDON — The U.K. is administering the first doses of the Moderna vaccine, the third authorized in the country against the coronavirus.

Patients at the West Wales General Hospital were receiving the jab on Wednesday. Britain has ordered 17 million doses of the Moderna vaccine, enough for 8.5 million people.

The rollout comes as the U.K. medical regulator investigates another vaccine, made by Oxford University and AstraZeneca, which has been given to more than 18 million people in Britain.

Several countries have restricted the AstraZeneca jab’s use in younger people while scientists investigate a small number of cases of rare blood clots in people who have received the vaccine.

Britain, which has ordered 100 million doses of the AstraZeneca shot, has not restricted its use, but its medical regulator is reviewing the evidence.

Oxford University said late Tuesday it had stopped giving the shot to children involved in a clinical trial until it had received more information from the regulator about reports of rare blood clots in adults.