The announcement came as the results from an earlier trial intended to prove the vaccine was safe and triggered antibody production was published.

The upcoming Phase 3 trial will recruit 30,000 participants in the US, with half to receive the vaccine at 100 microgram dose levels, and the other half to receive a placebo.

Researchers will then track them over two years to determine whether they are protected against infection by the virus. Or, if they do get infected, whether the vaccine prevents symptoms from developing.

If they do get symptoms, the vaccine can still be considered a success if it stops severe cases of COVID-19.

The study should run until 27 October 2022, but preliminary results should be available long before.

Moderna is considered to be in a leading position in the global race to find a vaccine against the coronavirus, which has infected more than 13.2 million people and killed 570,000.

But scientists caution that the first vaccines to come to market may not be the most effective or safest.

Moderna had previously published "interim results" from the first stages of its trial, called Phase 1 in May.

The early results were called "encouraging" by Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, which is co-developing the vaccine.

But some in the scientific community said they would reserve judgment until they saw the full results in peer-reviewed form.

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