(Reuters) - AstraZeneca Plc said it has paused 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 an emailed 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.

According to a New York Times report which cited a person familiar with the situation, a participant based in the United Kingdom was found to have transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections.

Whether the illness was directly linked to AstraZeneca’s vaccine remains unclear, the report said. AstraZeneca declined to comment on the report.

SHARES SLIDE AstraZeneca’s shares fell more than 8% in after-hours U.S. trading, while shares in rival vaccine developer Moderna Inc rose more than 4%. Shares in AstraZeneca’s Indian unit, AstraZeneca Pharma India Ltd, tumbled more than 12%.

Called AZD1222, the vaccine is in late-stage clinical trials in the United States, Britain, Brazil and South Africa and additional trials are planned in Japan and Russia. The trials aim to enroll up to 50,000 participants globally.

South Korea said on Wednesday it would look into the suspension and review its plan to participate in the manufacturing of the vaccine.

“It’s not rare that clinical trials get suspended as various factors interact,” health ministry official Yoon Tae-ho told a briefing.

The decision to put the trial on hold has impacted clinical trials being conducted by other vaccine makers, which are looking for signs of similar reactions, Stat said.

The U.S. National Institutes of Health, which is providing funding for AstraZeneca’s trial, declined to comment.

Moderna said in an emailed statement it was “not aware of any impact” to its ongoing COVID-19 vaccine study at this time.

Nine leading U.S. and European vaccine developers pledged on Tuesday to uphold scientific safety and efficacy standards for their experimental vaccines despite the urgency to contain the coronavirus pandemic.

The companies, including AstraZeneca, Moderna and Pfizer, issued what they called a “historic pledge” after a rise in concern that safety standards might slip in the face of political pressure to rush out a vaccine.

The companies said they would “uphold the integrity of the scientific process as they work towards potential global regulatory filings and approvals of the first COVID-19 vaccines.”

The other signatories were Johnson & Johnson, Merck & Co, GlaxoSmithKline, Novavax Inc, Sanofi and BioNTech.