U.S. pharmaceutical company Johnson & Johnson said Monday it had paused its COVID-19 vaccine trial because one of its participants had become sick.

"We have temporarily paused further dosing in all our COVID-19 vaccine candidate clinical trials, including the Phase 3 ENSEMBLE trial, due to an unexplained illness in a study participant," the company said in a statement.

The patient's condition was being reviewed and evaluated by the ENSEMBLE independent Data Safety Monitoring Board as well as internal clinical and safety physicians, it said.

Adverse events, even those that are serious, are an expected part of any clinical study, especially large studies, according to the company.

There are prespecified guidelines for all clinical studies, said the company. "These ensure studies may be paused if an unexpected serious adverse event (SAE) that might be related to a vaccine or study drug is reported, so there can be a careful review of all of the medical information before deciding whether to restart the study."

Johnson & Johnson said SAEs are not uncommon in clinical trials, and the number of SAEs can reasonably be expected to increase in trials involving large numbers of participants.

As many trials are placebo-controlled, it is not always immediately apparent whether a participant received a study treatment or a placebo, according to the company.

The phase 3 clinical trial of Johnson & Johnson's COVID-19 vaccine started in Sept. 23. Enditem