The vaccine maker said the regulator placed the hold because of a report of a “serious adverse event” of motor neuropathy in one clinical-trial participant outside the U.S.

While respiratory illnesses are in Arkansas year-round, the Arkansas Department of Health said they typically increase in the fall and peak in January.

The so-called clinical hold is due to a single report of nerve damage in a patient who received the combination shot in a phase two trial.

Novavax Inc.’s stock tumbled 16% early Wednesday after the company said the FDA has placed a clinical hold on its COVID-19-and-flu combination and stand-alone flu vaccine candidates, following a case of motor neuropathy in a participant in a mid-stage trial.