The US biotech firm Moderna said it would enter the final stage of human trials for its Covid-19 vaccine on July 27, after promising early results were published in an influential journal.
Early-stage testing showed the vaccine tested in the US revved up people's immune systems the way scientists had hoped.
"No matter how you slice this, this is good news," said Dr Anthony Fauci, the US government's top infectious disease expert.
The experimental vaccine, developed by Fauci's colleagues at the National Institutes of Health and Moderna Inc, will start its most important step around July 27: A 30,000-person study to prove if the shots really are strong enough to protect against the coronavirus. The study should run until October 27, 2022. It is designed to show whether the vaccine is safe and can prevent infection by the SARS-CoV-2 virus, or - if people still get infected - whether it can prevent the infection progressing towards symptoms.
If they do get symptoms, the vaccine can still be considered a success if it stops severe cases of Covid-19.