Late-stage COVID-19 vaccine trials by Johnson & Johnson have been temporarily halted due to an “unexplained illness in a study participant,” the company reported Monday.

The illness, discovered in the Phase 3 ENSEMBLE trial, is under review by an independent board. Study participants will not be dosed until further notice, and it will be at least “a few days at minimum,” before detailed information is provided by investigators, according to Reuters. It is not yet known whether the ill participant had been given a vaccine or placebo, as the study is blinded.

Patient privacy and further investigation preclude the release of more details, Johnson & Johnson said. 

“Adverse events — illnesses, accidents, etc. — even those that are serious, are an expected part of any clinical study, especially large studies,” the company stated.