The Moderna trial launch comes exactly one year after the first adult received a test dose of the shot.
Executives say the first age group to start in the trial will be children ages 6 to 11, followed by children ages 2 to 6 and then children 6 months to 2 years of age.
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Moderna's trial will enroll nearly 7,000 children from ages 6 months to 12 years in the U.S. and Canada, according to the company.
"There will be two parts to the study," Dr. Jacqueline Miller, a pediatrician and Moderna's therapeutic area head for infectious diseases, told "Good Morning America." "The first part is where we will find the appropriate dose of the vaccine."
"Children often need lower doses of vaccines than adults and we want to make sure we find the best dose that increases their immunity," she said.
Both the Moderna and Pfizer vaccines received emergency use authorization from the FDA in December. The two pharmaceutical companies have enrolled children 12 and older in clinical trials of their vaccines and hope to have results by the summer.
Johnson & Johnson, whose vaccine received emergency use authorization in February, has not yet announced plans to test its vaccine in children.
While children have not faced as many deaths from COVID-19 as adults, they can still get the virus and even more importantly, they can transmit the virus to adults.
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Last month, the Biden administration announced a revised timeline for children to receive COVID-19 vaccines, telling reporters at a Feb. 19 briefing that high school students may receive vaccines in the fall, but elementary school children won't receive them until early 2022.
Previously, Dr. Anthony Fauci, the White House chief medical adviser, predicted high school students would begin to get vaccinated in the spring or summer, while students in first grade would begin getting vaccinated by the time school returns in September. The new timeline pushes each stage back by around four to five months. The White House did not provide an explanation for the shift.
More than 70 million adults in the U.S. have now received at least one dose of a COVID-19 vaccine, or nearly 28% of the adult population, according to the U.S. Centers for Disease Control and Prevention (CDC). At least 22 states plus Washington, D.C., are already allowing people 16 years old and older who have high-risk medical conditions to receive a vaccine.
ABC News contributed to this report.