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Kids are now receiving their first doses of Moderna’s COVID-19 vaccine, as the company begins phases two and three of their new vaccine trials.

About 6,750 healthy children aged 6 months to 11 years will take part in this two-part 14-month study, dubbed KidCove. In the first segment, each child will receive two shots spaced out by 28 days—the same regimen that was first tested in adults last year—but they will receive doses of either 50 or 100 micrograms each. Children under the age of two will receive shots that are either 25, 50, or 100 micrograms. Adult doses are 100 micrograms, for both the initial shot and the booster.

The first part of the study will inform which dosing size will be used in the second part, where those children will either receive the vaccine or a placebo.

In addition to the shots, the study will involve regular telemedicine check-ins, and will require the kids and their parents to log any daily symptoms into a smartphone app.

“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’s Chief Executive Officer, Stéphane Bancel, said in the company’s news release.

Immunogenicity, which is one of the main goals of the study, is different from efficacy, the goal of the original adult studies. Rather, it studies the type of immune response the vaccine provides and how well it works over time.

Pediatricians have been urging officials that even though they’re a segment of the population less affected by Sars-CoV-2, children should not be left behind when studying how well a vaccine works.

[Related: How to prepare for getting the COVID-19 vaccine]

In a letter to the White House last month, Lee Savio Beers, the president of the American Academy of Pediatrics (AAP) wrote that “having a COVID-19 vaccine available for children is essential for our nation to end the pandemic” and that “we must make sure that vaccine trials in children are equitable and include those at increased risk who could most benefit from a vaccine, particularly Black and Latinx children.”

According to AAP data, more than 3.2 million children have been infected by Sars-CoV-2 in the U.S., and at least 266 have died.

Moderna gave their vaccine to adolescents aged 12-17 in a phase 3 clinical trial back in December. Those trial results are expected to come in quickly, by the end of the summer, giving parents an optimistic outlook for their teens to return to school in the fall. Given that the study in younger kids will still be underway, children under 12 are not likely to be vaccinated until 2022.

“The reason for that is…you do need to be a little more cautious in progressing and working down dose levels to find the right dose,” Stephen Hoge, the president of Moderna said on the Today Show. Things like sensitivity and side effects will be scrutinized more carefully in younger kids especially, and so naturally these trials will take a little longer.

Other companies are also on the move to test their COVID-19 vaccines in children. Right now, Pfizer and BioNTech are doing trials in children ages 12 to 15, and are currently planning their next trials on younger kids. AstraZeneca began testing its vaccine in kids aged 6 and older in the UK and Europe a month ago, and The New York Times reported that Johnson & Johnson will also be rolling out trials in older and then younger children.

Children under age 18 make up about a fourth of the US population, so ensuring the vaccines are safe and effective for them will be crucial as we continue to make our way out of the pandemic.