Moderna later said in a statement that the trial will include 600 participants.
The phase 2 trial is a “crucial step” toward potential full clearance of a first batch as early as 2021, the company said.
During an investor call this morning, Moderna said it plans to add people older than 55 to the phase 2 study.
It further said the first batches of the vaccine are set to be manufactured in July, with the goal to enable manufacturing of up to 1 billion doses per year.
The vaccine named mRNA-1273, which was developed by the company and the National Institute of Allergy and Infectious Diseases, used a genetic platform called mRNA.
According to the National Institutes of Health, the vaccine directs the body’s cells to express a virus protein that researchers hope will elicit an immune response. Moderna has never brought a product to market, or gotten any of its vaccine candidates approved for use by the FDA.
This is the first U.S. vaccine to start clinical trials in the United States.
Moderna added that it is finalising protocol for a phase 3 study, which is expected to begin in early summer.