The Cambridge, Massachusetts-based biotech said the primary goal of the study would be to prevent symptomatic COVID-19, the disease caused by the novel coronavirus. The key secondary goal would be prevention of severe disease, as defined by keeping people out of the hospital.
The company's shares jumped 6% in premarket trading.
Moderna said it has selected the 100-microgram dose of the vaccine for the late-stage study. At that dose level, the company is on track to deliver about 500 million doses per year, and possibly up to 1 billion doses per year, starting in 2021 from the company's internal US manufacturing site and strategic collaboration with Swiss drugmaker Lonza.
The company said it chose the 100-microgram dose to maximize the immune response and minimize adverse reactions.
Moderna said is has completed manufacturing of enough vaccine to start the phase 3 trial.
In the midstage study, the company said it has enrolled 300 healthy adults, who have each been dosed with at least one shot, as well as the first 50 older adults, aged 18 to 54.
Testing the vaccine in older adults with be critical because this group is at higher risk for the most severe effects of the virus, and older adults typically have less efficient immune function. The midstage study is testing the safety and preliminary effectiveness of two doses of the vaccine given 28 days apart.
Study participants will be followed for a year.