A screenshot taken from the website of the U.S. National Institutes of Health (NIH) on Sept. 1, 2020 shows the title and photo of its latest publication "COVID-19 Phase 3 Vaccine Trial Begins." (Xinhua)

"Safe and effective vaccines will be essential to meet the global need for widespread protection against COVID-19," says NIAID Director Anthony Fauci.

WASHINGTON, Sept. 1  -- Researchers across the United States are stepping up efforts to work out an effective COVID-19 vaccine as another vaccine candidate has started phase 3 clinical trial on Monday.

The multi-site clinical trial evaluating an investigational COVID-19 vaccine, known as AZD1222, will enroll approximately 30,000 adult volunteers at 80 sites in the United States to evaluate if the candidate vaccine can prevent COVID-19, said the U.S. National Institutes of Health (NIH).

Participants will be randomly assigned to the investigational vaccine group or the placebo group, and neither the investigators nor the participants will know who is assigned to which group.

After an initial screening, participants will receive two injections of either the investigational vaccine or a saline placebo approximately four weeks apart.

One person will receive a placebo injection for every two people who receive AZD1222, which will result in approximately 20,000 people receiving the investigational vaccine and 10,000 people receiving a placebo.