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.
The vaccine is a small snippet of the coronavirus's genetic code, which is injected into the patient.
It is not capable of causing an infection or the symptoms of Covid-19, but is enough to provoke a response from the immune system.
The vaccine trials, run by the US government's National Institute of Allergy and Infectious Diseases, showed the vaccine led to the production of antibodies which can neutralise the coronavirus.
However, testing for these neutralising antibodies has only taken place on the first eight, out of 45, people on the trial.