Johnson & Johnson said it had temporarily halted 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 pause means the enrollment system has been closed for the 60,000-patient clinical trial while the independent patient safety committee is convened, AFP reported.

J&J said that serious adverse events (SAEs), such as accidents or illnesses, are “an expected part of any clinical study, especially large studies.” Company guidelines allow them to pause a study to determine if the SAE was related to the drug in question and whether to resume study.