(Bloomberg) -- U.S. National Institutes of Health Director Francis Collins told a Senate committee Wednesday that AstraZeneca Plc’s closely watched Covid-19 vaccine trial had been halted due to a “spinal cord problem.”

The vaccine front-runner and its partner, the University of Oxford, stopped giving experimental shots after a person participating in one of their studies got sick, triggering a review of safety data.

Appearing before the Senate Committee on Health, Education, Labor & Pensions, Collins said that the severe adverse event in question was transverse myelitis, a neurological disorder that causes inflammation of the spinal cord. If AstraZeneca’s review finds the adverse event is related to the vaccine, all the doses already manufactured will be thrown away, Collins said.

“This ought to be reassuring to everybody listening when we say we are going to focus first on safety and make no compromises, this is exhibit A,” Collins told the Senate committee Wednesday. “This is based on a single severe adverse event which may or may not have anything to do with the vaccine, but it is the best sort of cautious approach to quickly stop and look and see if there’s any other evidence to be concerned about.”

AstraZeneca Chief Executive Officer Pascal Soriot told investors on Wednesday morning that studies of the company’s vaccine were paused worldwide after a woman in the U.K. started showing symptoms of a rare and devastating neurological condition called transverse myelitis, marking the second time someone in one of its vaccine trials developed a central nervous system problem, according to a report published by STAT.

The woman is recovering and her diagnosis hasn’t been finalized, while the first case of neurological symptoms were deemed to be caused by multiple sclerosis and unrelated to the vaccine, the news organization reported.

AstraZeneca could resume its clinical trial next week, the Financial Times reported, citing unnamed people familiar with the matter.

The experimental vaccine being developed by AstraZeneca has been viewed as one of the leading candidates in the race to bring a shot to market. The partners are aiming to enroll as many as 50,000 participants for late-stage trials that are underway in the U.K., U.S. and other countries around the world.

The top U.S. official in charge of the White House’s Operation Warp Speed effort said experts monitoring the trials in the U.K. paused the late-stage trial in coordination with their U.S. counterparts.

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