NEW YORK — AstraZeneca  has suspended global trials, including large late-stage trials, of its experimental coronavirus vaccine due to an unexplained illness in a study participant.

The vaccine, developed with the University of Oxford, has been described by the World Health Organization as probably the world’s leading candidate and the most advanced in terms of development. The suspension dims prospects for an early rollout amid reports that the United States was aiming for a fast-track approval before the November presidential election.

The British drugmaker said it voluntarily paused trials to allow an independent committee to review safety data, and it was working to expedite the review 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 a statement.

The nature of the illness was not disclosed, although the participant is expected to recover, according to Stat News, which first reported the suspension due to a “suspected serious adverse reaction”.

The U.S. Food and Drug Administration defines an adverse event as one in which evidence suggests a possible relationship to the drug being tested.