The U.K. Medicines Health Regulatory Authority recommended that the study resume after an independent review of the safety data triggered a pause on Sept. 6, Oxford said in a statement. It declined to disclose details about the volunteer’s illness.

While temporary halts are common in vaccine trials, the interruption to the closely watched Astra-Oxford study had raised concerns about the viability of one of the fastest-moving experimental shots seeking protection from the pandemic. The race to develop a Covid-19 vaccine has compressed what is normally a decade-long process into a matter of months, with data from final-stage trials expected as soon as next month.

Statements from Astra and Oxford on Saturday didn’t say anything about the status of tests outside the U.K. Trials of the Oxford vaccine were underway in the U.S., Brazil, South Africa and India before being paused after the safety review.

A representative for the U.S. National Institutes of Health couldn’t immediately be reached for comment. An AstraZeneca spokesman declined to comment.

Suspected Diagnosis

AstraZeneca Chief Executive Officer Pascal Soriot said on Thursday that the vaccine could still be available by the end of the year. An independent safety board was reviewing whether the participant’s illness had been caused by the vaccine or was unrelated, he said.

Soriot said it wasn’t clear whether the participant had a condition called transverse myelitis, a suspected diagnosis. NIH Director Francis Collins told a Senate committee Wednesday the trial had been halted due to a “spinal cord problem.”

“We cannot disclose medical information about the illness for reasons of participant confidentiality,” Oxford said. “We are committed to the safety of our participants and the highest standards of conduct in our studies and will continue to monitor safety closely.”

Oxford said some 18,000 people have received “study vaccines” as part of the trials. It had begun a large phase 3 trial in the U.S. at the end of August, with the aim of enrolling 30,000 people.

AstraZeneca is one of several companies taking part in the U.S. government’s Operation Warp Speed program to fast-track a coronavirus vaccine. In May, the company inked a $1.2 billion deal with the U.S. to support clinical studies and supply 300 million doses of the vaccine. It has pledged to provide the vaccine on a not-for-profit basis during the pandemic and has lined up deals around the world to supply almost 3 billion doses.

Human Trials

Oxford began human trials of its shot on April 23 in almost 1,100 volunteers and at the end of May advanced quickly to a large-scale study in the U.K. involving more than 10,000 people including those over the age of 55. Researchers are looking for a significantly higher number of Covid-19 cases in the control group over the vaccinated group to show the shot is effective.

The trial started just as rates of infection in the U.K. began dropping in May, making it harder to demonstrate whether the vaccine works. In recent weeks, new cases have been climbing again.

In July, Oxford reported initial results showing the vaccine increased levels of protective neutralizing antibodies and immune T-cells that target and destroy infected cells. It didn’t report any serious adverse events as part of its phase 1-2 trial. Researchers pressed ahead with trialing a two-dose regime after finding it produced a stronger response in 10 volunteers. It’s unclear whether the participant who fell ill had two doses or one.