(CNN) -- Johnson & Johnson has paused its COVID-19 vaccine clinical trial because of an unexpected illness.
The drugmaker says the participant is being evaluated by an independent safety monitoring board, as well as physicians within the company.
No other details about the illness were released.
Johnson & Johnson says this is only a study pause, which is standard protocol during a clinical trial.
Trials are meant to see if vaccines cause dangerous side effects.
When an illness occurs, the trial stops so doctors can check if it is linked to the vaccine or just a coincidence.
This is the second Phase 3 coronavirus vaccine trial to be paused in the US.
Astrazeneca's vaccine trial was paused last month because of a neurological complication in a volunteer in Britain.
The trial has resumed in other countries, but it's still paused in the US as the Food and Drug Administration investigates.