Moderna said neutralising antibodies were found in the first eight people who took part in their safety trials.
It also said the immune response was similar to that in people infected with the actual virus.
Larger trials to see whether the jab protects against infection are expected to start in July.
Work on a coronavirus vaccine has been taking place at unprecedented speed, with around 80 groups around the world working on them.
Moderna was the first to test an experimental vaccine, called mRNA-1273, in people.