Originally Published: 21 JUN 22 08:09 ET

Updated: 21 JUN 22 12:31 ET

By Jacqueline Howard, CNN

(CNN) -- Dr. Sarah Schaffer DeRoo described in one word how she felt after getting her 7-month-old son vaccinated against Covid-19: thrilled.

Her active baby boy sat in her lap at a vaccine clinic hosted by Children's National Hospital in Washington, DC, while receiving his first dose of Covid-19 vaccine. The shot was administered in his thigh. He cried for a few seconds but then his attention turned toward a golden retriever that was on site as a comfort dog provided by the hospital.

"I'm feeling really thrilled that we have this opportunity," DeRoo, a pediatrician at Children's National Hospital in Washington, DC, told CNN's Suzanne Malveaux on Tuesday about her son's vaccination.

DeRoo added that her family now will feel more comfortable participating in certain activities, knowing that their youngest son has started his Covid-19 vaccine series.

"It will certainly allow us to have more freedom with our personal lives and what we do," DeRoo said. "And for the baby, we'll feel like we have cloaked him in as much protection as we can."

Covid-19 vaccinations for children younger than 5 are beginning Tuesday across the United States, marking a milestone in the nation's fight against the disease.

Last week, the US Food and Drug Administration expanded the emergency use authorizations for Moderna's vaccine to include children 6 months through 17 years and Pfizer/BioNTech's for children 6 months through 4 years.

Then on Saturday, US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky signed off on Covid-19 vaccinations for children under 5, clearing the way for vaccinations to be administered in that age group.

About 17 million kids under the age of 5 are now eligible for Covid-19 vaccines.

"This is a big day. We've been waiting a long time for children to have access to the vaccine. We now have every age group, 6 months and above, in the country which is now eligible to get protection from the Covid-19 vaccine. And I'll tell you as a dad of a 4-year-old, this is a big deal for my family as well," US Surgeon General Dr. Vivek Murthy told CNN's Brianna Keilar on Tuesday morning.

Vaccines given in child-sized doses

Under the FDA's authorization, the Moderna vaccine can be given as a two-dose primary series, with doses given four weeks apart, at 25 micrograms each dose, to infants and children 6 months through 5 years of age.

While the FDA has authorized Moderna's vaccine for children ages 6 to 17, the CDC has not yet recommended it for that age group, so those shots can't be administered yet. The FDA authorization would allow children ages 6 to 11 to receive doses are 50 micrograms each. For those ages 12 and older, it would be administered as 100-microgram doses.

The Pfizer/BioNTech vaccine now can be given as a three-dose primary series, at 3 micrograms each dose, for use in infants and children 6 months through 4 years. The vaccine is administered as a two-dose primary series at 10 micrograms per dose for children 5 to 11 and at 30 micrograms per dose for adolescents and adults ages 12 and older.

Completing the Pfizer/BioNTech vaccine series is a longer process, as the first two doses are administered three weeks apart, and then the third dose is given eight weeks later.

Dr. Jeannette Lee of the University of Arkansas for Medical Sciences, who serves on the FDA's vaccine advisory committee, expressed concern about children not completing all three doses.

"Three doses will certainly benefit. I have a lot of concern that many of these kids will not get a third dose," Lee said. "My concern is that you have to get the three doses to really get what you need."

As for children who might turn from age 4 to 5 at any point while completing their Pfizer/BioNTech vaccine series, the CDC recommends two options. The child could complete the two-dose primary series authorized for children ages 5 to 11, or they could complete the three-dose series for younger kids, but each of doses 2 and 3 may be either the dosage for younger children or ages 5 to 11.

The FDA's Vaccines and Related Biological Products Advisory Committee determined that the benefits of both vaccines outweigh the risks and noted that the vaccines have been "well-tolerated" among the children who got them in clinical trials.

According to clinical trial data, common side effects for both vaccines include pain at the injection site, headache, fever, chills and fatigue. The vaccines appeared to elicit similar immune responses in children as has been seen in adults.

Where young children can get vaccinated

Pediatricians' offices and pharmacies are the main sites where young children could get vaccinated.

"We know that parents are going to want to get their children vaccinated in pediatricians' offices. Some people will go to a pharmacy, some people will go to a children's hospital or some sort of a community health center," Dr. Ashish Jha, the White House's Covid-19 response coordinator, said Monday on CBS.

"But the bottom line is, I think a majority of parents are going to want to get their child vaccinated in their pediatrician's office," Jha said. "So, many pediatricians are going to be offering the vaccine."

As for the pharmacy locations offering these child-sized vaccines, CVS and Walgreens have announced plans to provide vaccinations.

CVS will begin administering Pfizer's Covid-19 vaccines to children under five on Tuesday, a communications representative told CNN.

"We will begin administering the Pfizer-BioNTech Covid-19 vaccine for eligible children 18 months through four years of age at our 1,100 MinuteClinic locations starting on Tuesday," Matt Blanchette, senior manager of retail communications with CVS Pharmacy, told CNN in an email.

"MinuteClinic is located inside select CVS Pharmacy stores in 35 states and Washington, DC," Blanchette said. Appointments will be available on a rolling basis according to vaccine supply.

Blanchette said children over 5 will still be able to access Covid-19 vaccines in CVS pharmacies.

On Saturday, Walgreens announced in a news release that appointments for vaccinations in young children will be available starting June 25. Walgreens will be vaccinating children 3 and older at "select" locations, and appointments can be scheduled online.

Hy-Vee pharmacies will have Covid-19 vaccinations available for children under 5 once doses are available, a communications representative told CNN on Monday.

"Hy-Vee anticipates receiving its allocations for the newly approved age groups in the coming days," Tina Potthoff, senior vice president of communications, wrote in an email.

"As soon as we receive vaccine and our appointment scheduler is open for these age groups, we will post an update on our COVID-19 vaccine landing page, post on our Hy-Vee store Facebook pages, and contact media outlets in our eight-state region to make them aware of our pediatric hubs that are accepting appointments."

Due to federal regulations, she wrote, Hy-Vee will only be providing vaccinations to children 3 and older.

"Under the PREP Act, retail pharmacies, including Hy-Vee pharmacies, are only authorized to administer vaccines to patients ages 3+," Potthoff wrote in the email. "Patients younger than age 3 should visit their pediatrician or health care provider to receive the COVID-19 vaccine."