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.

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 on Sept. 23. The trial is designed to evaluate if the vaccine candidate could prevent symptomatic COVID-19 after a single dose regimen.

Up to 60,000 volunteers are scheduled to be enrolled in the trial at up to nearly 215 clinical research sites in the United States and internationally.

It is the fourth large-scale phase 3 clinical trials for a COVID-19 vaccine in the United States.

The other three trials are for vaccine candidate AZD1222, co-invented by the University of Oxford and its spin-out company Vaccitech; vaccine candidate mRNA-1273, developed by the U.S. National Institute of Allergy and Infectious Diseases and American biotechnology company Moderna; and vaccine candidate BNT162b2, developed by American biopharmaceutical company Pfizer and German company BioNTech.

Johnson & Johnson's vaccine wing Janssen is responsible for developing the shot. While the other vaccine candidates require two doses, the Janssen vaccine candidate will be studied as a single-dose vaccine.

It is a recombinant vector vaccine that uses a human adenovirus to express the SARS-CoV-2 spike protein in cells.