The Food and Drug Administration's action follows its advisory panel's unanimous recommendation for the shots from Moderna and Pfizer. That means U.S. kids under 5 - roughly 18 million youngsters - are eligible for the shots, about 1 1/2 years after the vaccines first became available in the U.S. for adults, who have been hit the hardest during the pandemic.
There's one step left: The Centers for Disease Control and Prevention recommends how to use vaccines. Its independent advisers began debating the two-dose Moderna and the three-dose Pfizer vaccines on Friday and will make its recommendation Saturday. A final signoff would come from CDC Director Dr. Rochelle Walensky.
At a Senate hearing Thursday, Walensky said her staff was working over the Juneteenth federal holiday weekend "because we understand the urgency of this for American parents."
She said pediatric deaths from COVID-19 have been higher than what is generally seen from the flu each year.
"So I actually think we need to protect young children, as well as protect everyone with the vaccine and especially protect elders," she said.
MORE: FDA committee clears the way for kids 6-17 to get another COVID vaccine option in Moderna
The FDA also authorized Moderna's vaccine for school-aged children and teens. Pfizer's shots had been the only option for those ages.
For weeks, the Biden administration has been preparing to roll out the vaccines for little kids, with states, tribes, community health centers and pharmacies preordering millions of doses. FDA's emergency use authorization allows manufacturers to begin shipping vaccine across the country. Vaccinations could begin early next week.
While young children generally don't get as sick from COVID-19 as older kids and adults, their hospitalizations surged during the omicron wave and FDA's advisers determined that benefits from vaccination outweighed the minimal risks. Studies from Moderna and Pfizer showed side effects, including fever and fatigue, were mostly minor.
"As we have seen with older age groups, we expect that the vaccines for younger children will provide protection from the most severe outcomes of COVID-19, such as hospitalization and death," FDA Commissioner Robert Califf said in a statement.
In testing, the littlest children developed high levels of virus-fighting antibodies, comparable to what is seen in young adults, the FDA said. Moderna's vaccine was about 40% to 50% effective at preventing infections but there were too few cases during Pfizer's study to reliably determine effectiveness, the agency said.
''Both of these vaccines have been authorized with science and safety at the forefront of our minds,'' said Dr. Peter Marks, FDA's vaccine chief.
The two brands use the same technology but there are differences.
Pfizer's vaccine for kids younger than 5 is one-tenth of the adult dose. Three shots are needed: the first two given three weeks apart and the last at least two months later.
Moderna's is two shots, each a quarter of its adult dose, given about four weeks apart for kids under 6. The FDA also authorized a third dose, at least a month after the second shot, for children who have immune conditions that make them more vulnerable to serious illness.
Both vaccines are for children as young as 6 months. Moderna next plans to study its shots for babies as young as 3 months. Pfizer has not finalized plans for shots in younger infants. A dozen countries, including China, already vaccinate kids under 5, with other brands.
In the U.S., it remains uncertain how many parents want their youngest vaccinated. By some estimates, three-quarters of all children have already been infected. And only about 30% of children aged 5 to 11 have been vaccinated since Pfizer's shots opened to them last November.
Dr. Beth Ebel, professor of pediatrics at University of Washington in Seattle, said the tot-sized vaccines would be especially welcomed by U.S. parents with children in daycare where outbreaks can sideline parents from jobs, adding to financial strain.
"A lot of people are going to be happy and a lot of grandparents are going to be happy, too, because we've missed those babies who grew up when you weren't able to see them," Ebel said.