Johnson & Johnson’s COVID-19 vaccine trial has been paused due to an unexplained illness in a volunteer. Now two of the four vaccine trials in the United States are now on hold…

Johnson & Johnson said Monday it has paused further dosing in its trial while the participant’s illness is reviewed and evaluated by an independent Data Safety Monitoring Board as well as the company’s clinical and safety physicians.

In any study the size of the Johnson & Johnson one, which will include up to 60,000 people when it is fully enrolled, some adverse events are expected among the participants. Such studies have clear, prespecified guidelines that say what events would trigger a shutdown.

The independent Data Safety Monitoring Board, which oversees the trial, will review what happened and decide whether it is safe to resume the trial.