A medical worker instructs a woman to take a swab sample at a COVID-19 drive-thru testing site in Atlanta, Georgia, the United States, July 20, 2020. (Photo by Alan Chin/Xinhua)

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.

WASHINGTON, Sept. 23 (Xinhua) -- 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.