Shares of French drugmaker Valneva slumped more than 20% on Monday after the U.S. Food and Drug Administration suspended the licence for its chikungunya vaccine, Ixchiq, with immediate effect, citing serious safety concerns.

Valneva must immediately cease U.S. shipments and sales of Ixchiq, a vaccine for chikungunya virus. The FDA recently lifted a pause on dosing of Ixchiq, but the agency’s new safety alert is based on an assessment that found the vaccine appears to be causing chikungunya-like illness in recipients.

The FDA suspends Ixchiq, the only approved chikungunya vaccine, over safety concerns, impacting its availability in the U.S. market.