Moderna and the National Institute of Allergy and Infectious Diseases began phase three trials for the coronavirus vaccine on Monday.

They will conduct phase-three trials at nearly 100 U.S. research sites with 30,000 adult volunteers. Volunteers will receive two 100 microgram injections of the vaccine and the control group will be injected with placebo about 28 days apart. The first patient received a vaccine at a site in Savannah, Georgia.

“We are pleased to have started the Phase 3 COVE study,” Moderna CEO Stéphane Bancel said in a statement. “We are grateful to the efforts of so many inside and outside the company to get us to this important milestone. We are indebted to the participants and investigators who now begin the work of the COVE study itself. We look forward to this trial demonstrating the potential of our vaccine to prevent COVID-19, so that we can defeat this pandemic.”

Phase 1 trial results of the vaccine were published earlier this month in the New England Journal of Medicine. They showed that the vaccine produced an immune response in all volunteers and was deemed generally safe, with non-serious side effects, including fatigue, chills, headache and muscle pain. None of the side effects required hospitalization.

The Moderna/NIH vaccine becomes the first U.S. candidate to reach Phase 3. The vaccine is among 25 others in clinical trials around the world, according to the World Health Organization.