WASHINGTON - A fourth Phase 3 clinical trial evaluating an investigational COVID-19 vaccine has begun enrolling adult volunteers, the U.S. National Institutes of Health (NIH) announced on Wednesday.

The trial is designed to evaluate if the investigational Janssen COVID-19 vaccine JNJ-78436725 can prevent symptomatic COVID-19 after a single dose regimen, the NIH said in a release.

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

The Janssen Pharmaceutical Companies of Johnson & Johnson developed the investigational vaccine, and is leading the clinical trial as regulatory sponsor.

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 (NIAID) and American biotechnology company Moderna; and vaccine candidate BNT162b2, developed by American biopharmaceutical company Pfizer and German company BioNTech.

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.

Preclinical findings published in Nature show that the Janssen vaccine induced neutralizing antibody responses in rhesus macaques and provided complete or near-complete protection against virus infection in the lungs and nose following SARS-CoV-2 challenge.

The trial is designed primarily to determine if the investigational vaccine can prevent moderate to severe COVID-19 after a single dose. It also aims to understand if the vaccine can prevent COVID-19 requiring medical intervention and if the vaccine can prevent milder cases of COVID-19 and asymptomatic SARS-CoV-2 infection, according to the NIH.

An independent Data and Safety Monitoring Board will provide oversight to ensure the safe and ethical conduct of the study.

"Four COVID-19 vaccine candidates are in Phase 3 clinical testing in the United States just over eight months after SARS-CoV-2 was identified. This is an unprecedented feat for the scientific community made possible by decades of progress in vaccine technology and a coordinated, strategic approach across government, industry and academia," said NIAID Director Anthony Fauci.

"It is likely that multiple COVID-19 vaccine regimens will be required to meet the global need. The Janssen candidate has showed promise in early-stage testing and may be especially useful in controlling the pandemic if shown to be protective after a single dose," Fauci said.