(Bloomberg) -- Novavax Inc. shares fell 19% after US regulators placed a hold on the company’s experimental vaccines because a study volunteer developed a serious nerve disorder.

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The pause applies to the company’s stand-alone influenza shots and the influenza and Covid combination vaccine, Novavax said in a statement. Chief Medical Officer Robert Walker said the company doesn’t believe causality has been established and it’s working with regulators to resolve the issue.

The volunteer who developed motor neuropathy, which led to problems with movement, had received a combination flu and Covid shot in January 2023 as part of a study taking place outside the US. The trial was completed in July 2023, and at that point there was no indication of a complication, Walker said in an interview.

As soon as Novavax received the report of the adverse event late last month, it notified the US Food and Drug Administration, he said. There was no indication of serious adverse events in anyone else, he said.

Novavax and FDA will be working with the participant’s health care team to get more details, including when exactly the health issues began.

“We will continue to investigate,” Walker said. “Our first priority is protecting the safety of the participants in the study.”

The FDA declined to comment.

Novavax came to prominence after winning approval for a Covid-19 vaccine during the height of the pandemic, though it faced a rockier path than rivals Pfizer Inc. and Moderna Inc. Last year, executives raised doubt about the company’s ability to stay in business.

It seemed to turn a corner in May, signing a $1.2 billion licensing agreement with Sanofi that included commercializing a combination Covid-flu shot.

The company’s protein-based approach looked to be an effective option during the pandemic when the need for immunizations was widespread. Despite good trial results and emergency clearance from the FDA, manufacturing delays limited its use.

(Updates shares in the third paragraph, and adds details from interview with chief medical officer in the fourth paragraph)

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