CAMBRIDGE (CBS) – Moderna has started a study of its coronavirus vaccine in children under the age of 12.

The Cambridge-based company announced Tuesday that the first participants in the research – ages 6 months to less than 12 years – have received doses.

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The two-part study is designed to show how children tolerate the vaccine, and to make sure it’s safe and effective for them.  This expands the age bracket of Moderna’s study which started with kids ages 12 to 17.

Moderna’s vaccine is two shots given 28 days apart.

The study, called kidCOVE, is expected to enroll 6,750 healthy children in the U.S. and Canada. As with earlier studies, some will receive the vaccine and others will get a placebo. Moderna said the placebo is a “saltwater solution that looks just like the study vaccine but contains no active vaccine.”

The children will be followed for a year after their second vaccination.

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CBS News medical contributor Dr. David Agus said getting kids vaccinated is critical to ending the pandemic.

“We have to do everything we can to get to herd immunity and, given that 25-percent of the United States is under the age of 21, this is going to have to include children,” he said.

Moderna’s adult vaccine trial had 30,000 people. Agus said the smaller trial for children only needs to answer two questions.

“Is it safe? Period. And do they mount effective immune responses? Period. And those are very easy to do with smaller numbers of children.”

“It is humbling to know that 17.8 million adults in the U.S. have received the Moderna COVID-19 Vaccine to date. We are encouraged by the primary analysis of the Phase 3 COVE study of mRNA-1273 in adults ages 18 and above and this pediatric study will help us assess the potential safety and immunogenicity of our COVID-19 vaccine candidate in this important younger age population.” Moderna CEO Stéphane Bancel said in a statement.

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For more information about the trial and enrollment visit kidcovestudy.com.