Researchers across the US have stepped up efforts towards the development of an effective Covid-19 vaccine, as another vaccine candidate has started phase 3 clinical trial, according to the National Institutes of Health (NIH).

The multi-site clinical trial evaluating an investigational vaccine, known as AZD1222, will enroll approximately 30,000 adult volunteers at 80 sites in the US to evaluate if the candidate vaccine can prevent Covid-19, Xinhua news agency quoted the NIH as saying on Monday.

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.

The trial primarily is designed to determine if AZD1222 can prevent symptomatic Covid-19 after two doses.

The trial will also evaluate if the vaccine candidate can prevent the novel coronavirus infection regardless of symptoms and if it can prevent severe Covid-19.

It also further assess if the experimental vaccine can reduce the incidence of emergency department visits due to Covid-19, according to the NIH.

The UK-based global biopharmaceutical company AstraZeneca is leading the trial as regulatory sponsor.

The National Institute of Allergy and Infectious Diseases (NIAID), part of the NIH, and the Biomedical Advanced Research and Development Authority are providing funding support for the trial.

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

"Positive results from preclinical research led by NIH scientists supported the rapid development of this vaccine candidate, which has also showed promise in early-stage clinical trials."

Last month, the NIH announced the start of a randomized, controlled clinical trial evaluating the safety and efficacy of a treatment regimen consisting of the antiviral remdesivir plus the immunomodulator interferon beta-1a in COVID-19 patients.

The trial was anticipated to enroll more than 1,000 hospitalized adults with Covid-19 at as many as 100 sites in the US and abroad.

Another experimental Covid-19 vaccine being developed by the NIAID and American biotechnology company Moderna, known as mRNA-1273, started phase 3 clinical trial in July to evaluate if it can prevent Covid-19 in adults.

The trial, scheduled to be conducted at US clinical research sites, was expected to enroll approximately 30,000 adult volunteers who do not have Covid-19.