Johnson & Johnson has paused the large late-stage clinical trial of its coronavirus vaccine because of an “unexplained illness” in one of the volunteers, the company said on Monday.

The company did not say whether the sick participant had received the experimental vaccine or a placebo. The pause was first reported by the health news website Stat.

Johnson & Johnson, which just began the so-called Phase 3 trial of its vaccine last month, was behind several of its competitors in the vaccine race, but its vaccine had some advantages over others. It does not need to be frozen, and it could need just one dose instead of two. It would also be the largest trial, with a goal of enrolling 60,000 volunteers.

“Adverse events — illnesses, accidents, etc. — even those that are serious, are an expected part of any clinical study, especially large studies,” the company said in a statement. “We’re also learning more about this participant’s illness, and it’s important to have all the facts before we share information.”

Trial pauses are often lifted quickly after the illness is investigated and deemed not to be a serious safety risk.

“It’s actually a good thing that these companies are pausing these trials when these things come up,” said Dr Phyllis Tien, an infectious disease physician at the University of California, San Francisco. “We just need to let the sponsor and the safety board do their review and let us know their findings.”

“It doesn’t mean that the adverse event is related to the vaccine, but it needs to be investigated thoroughly,” said Dr Luciana Borio, who oversaw health preparedness for the National Security Council.