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A large, Phase 3 study testing a Covid-19 vaccine being developed by AstraZeneca and the University of Oxford at dozens of sites across the U.S. has been put on hold due to a suspected serious adverse reaction in a participant in the United Kingdom.

A spokesperson for AstraZeneca, a frontrunner in the race for a Covid-19 vaccine, said in a statement that the company’s “standard review process triggered a pause to vaccination to allow review of safety data.”

It was not immediately clear who placed the hold on the trial, though it is possible it was placed voluntarily by AstraZeneca and not ordered by any regulatory agency. The nature of the adverse reaction and when it happened were also not immediately known, though the participant is expected to recover, according to an individual familiar with the matter.

The spokesperson described the pause as “a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials, while it is investigated, ensuring we maintain the integrity of the trials.” The spokesperson also said that the company is “working to expedite the review of the single event to minimize any potential impact on the trial timeline.”

An individual familiar with the development said researchers had been told the hold was placed on the trial out of “an abundance of caution.” A second individual familiar with the matter, who also spoke on condition of anonymity, said the finding is having an impact on other AstraZeneca vaccine trials underway — as well as on the clinical trials being conducted by other vaccine manufacturers.

Clinical holds are not uncommon, and it’s unclear how long AstraZeneca’s might last. But the progress of the company’s trial — and those of all Covid-19 vaccines in development — are being closely watched given the pressing need for new ways to curb the global pandemic. There are currently nine vaccine candidates in Phase 3 trials. AstraZeneca’s is the first Phase 3 Covid-19 vaccine trial known to have been put on hold.

Researchers running other trials are now looking for similar cases of adverse reactions by combing through databases reviewed by a so-called Data and Safety Monitoring Board, the second person said.

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A person familiar with the situation, and who spoke on the condition of anonymity, said that the participant had been enrolled in a Phase 2/3 trial based in the United Kingdom. The individual also said that a volunteer in the U.K. trial had been found to have transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections. However, the timing of this diagnosis, and whether it was directly linked to AstraZeneca’s vaccine, is unclear.