The US Food and Drug Administration has granted emergency authorisation for the use of Pfizer and Moderna Covid-19 vaccines in the youngest children, the final age group awaiting immunisation in most countries.
The agency authorised Moderna's two-dose vaccine for children aged six months to five years, and three doses of Pfizer's shots for those between six months and four years old.
"Many parents, caregivers and clinicians have been waiting for a vaccine for younger children and this action will help protect those down to six months of age," Food and Drug Administration chief Robert Califf said in a statement.
"We expect that the vaccines for younger children will provide protection from the most severe outcomes of Covid-19, such as hospitalisation and death."
"Although the number of deaths in children is small by adult standards, any death of a child is tragic, and should be prevented if possible," Mr Califf said at a press conference.
"By vaccinating our youngest children, we hope to prevent the most devastating consequences of Covid," he added.
The Centers for Disease Control and Prevention (CDC) must now also recommend the vaccines before they are put into use - a final green light that will be given after a meeting of an advisory committee of experts that is expected to be held shortly.
But the US government has said that as soon as the FDA decision is made, ten million doses could immediately be sent around the country, followed by millions more in subsequent weeks.
The vaccines could be rolled out for the under-5 age groups as early as next week, White House officials have said, and pharmacy chains have conveyed that they are ready to distribute the shots.
Both vaccines are based on messenger RNA, which delivers genetic code for the coronavirus spike protein to human cells that then grow it on their surface, training the immune system to be ready.
The technology is now considered the leading Covid vaccination platform.
The vaccines were tested in trials of thousands of children. They were found to cause similar levels of mild side effects as in older age groups and triggered similar levels of antibodies.
Efficacy against infection was higher for Pfizer, with the company placing it at 80%, compared to Moderna's estimates of 51% for children aged six-months to two years old and 37% for those aged two to five years.
But the Pfizer figure is based on very few cases and is thus considered preliminary.
It also takes three doses to achieve its protection, with the third shot given eight weeks after the second, which is given three weeks after the first.
Moderna's vaccine should provide strong protection against severe disease after two doses, given four weeks apart, and the company is studying adding a booster that would raise efficacy levels against mild disease.
However, Moderna's decision to go with a higher dose is associated with higher levels of fevers in reaction to the vaccine compared to Pfizer.
There are some 20 million children aged four years and under in the United States.
While many American parents are eager to vaccinate their children, its unclear how strong the demand will be for the shots.
The Pfizer/BioNTech vaccine was authorised for children ages 5 to 11 in October, but only about 29% of that group is fully vaccinated, according to federal data.
"Those trusted with the care of children can have confidencein the safety and effectiveness of these COVID-19 vaccines andcan be assured that the agency was thorough in its evaluation ofthe data," Mr Califf said.
Public health officials and experts say that even though alarge portion of small children were infected during the winter surge due to the Omicron variant of the coronavirus, natural immunity wanes over time and vaccinations should help prevent hospitalisations and deaths when cases rise again.