AstraZeneca has said it has paused global trials, including large late-stage trials, of its experimental coronavirus vaccine because of an unexplained illness in a study participant.

The vaccine, developed with the University of Oxford, has been widely seen as one of the leading global candidates against the coronavirus, and the suspension of the trial dims prospects for a potential year-end rollout its lead developer had signalled earlier.

According to Reuters, AstraZeneca said it voluntarily paused trials to allow review of safety data by an independent committee and was working to expedite the review of the single event to minimise any potential impact on the trial timeline.

“This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials,” the company said in an emailed statement.