(Bloomberg) -- AstraZeneca Plc’s Covid-19 vaccine study remains paused in the U.S. pending a regulatory review of an incident in which a U.K. study participant became ill, federal officials said.

The British drugmaker and its partner, the University of Oxford, put research into the shot on hold last week after the volunteer suffered neurological symptoms. Trials resumed in the U.K. over the weekend and restarted in South Africa on Tuesday.

Temporary pauses in clinical trials aren’t unusual. However, the reported event has heightened worry that the global campaign to find a Covid-19 vaccine is moving too quickly. AstraZeneca and the University Oxford are among several drugmakers who have agreed to work with and supply shots to Operation Warp Speed, the Trump administration’s effort to expedite the development of an inoculation.

Moncef Slaoui, who is leading the Warp Speed initiative, said that the U.S. trial would resume when independent safety monitors and the Food and Drug Administration determine it is safe.

“This pause while the adverse event is fully investigated means the science-based, data-driven process is working as it should,” Slaoui said in a statement.

FDA Commissioner Stephen Hahn said during an Instagram Live event that the agency is “going to do very significant work with the company to figure out if there’s a significant safety issue or not.”

A representative for AstraZeneca declined to comment.

Data from final-stage trials of the AstraZeneca vaccine are expected as soon as next month, and Chief Executive Officer Pascal Soriot said last week the injection could still be available by the end of the year.

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