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The U.S. Food and Drug Administration has put on hold a trial of Novavax's COVID-influenza and its standalone flu vaccines after a participant who took the combination shot reported nerve damage, the company said on Wednesday.

Its shares plunged as much as 24 percent, and if losses hold, it would wipe off nearly $400 million from the vaccine maker's market capitalization.

The stock drop also reflects concerns on whether the hold would impact Novavax's partnership with French drugmaker Sanofi, according to one analyst.

Novavax said a participant enrolled in a mid-stage study of the combination vaccine last month reported symptoms of motor neuropathy, or damage to the nerve cells that control muscles or movement. The person was given the vaccine in January last year.

The company said it was not yet established that the vaccine had caused the safety event and it was working with the FDA to resolve the pause. Head over to Medscape to read the full story.