Moderna (MRNA) said Friday its trial for a trivalent vaccine against norovirus is currently on hold by the US Food and Drug Administration after a single adverse event report of a close of Guillain-Barre syndrome.

Investigation of the adverse event is currently underway, according to the company.

Moderna said it expects the clinical hold will not impact the efficacy readout timeline for the northern hemisphere trial as enrollment has already been completed.

Shares were up nearly 4% in recent trading.

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