US health authorities on Saturday cleared the Pfizer and Moderna Covid-19 vaccines for children aged five and younger, in a move President Joe Biden greeted as a "monumental step" in the fight against the virus.

The United States becomes the first country to approve use of the so-called mRNA vaccines for children as young as six months.

The US Food and Drug Administration (FDA) had on Friday authorised their emergency use for young children, who previously had to be at least five to be vaccinated.

But the vaccines required further clearance from the US Centers for Disease Control and Prevention (CDC), the country's leading public health agency – and they received that on Saturday.

"We know millions of parents and caregivers are eager to get their young children vaccinated, and with today's decision, they can," CDC Director Rochelle Walensky said in a statement Saturday.

Once the green light was received from the FDA, the US government began distributing millions of doses of the vaccine across the country.

Biden promised that parents could begin scheduling appointments as early as this week to have their young children vaccinated at hospitals, clinics, pharmacies and doctor's offices.

In a statement on Saturday, he touted the vaccines as "safe (and) highly effective," and said that "for parents all over the country, this is a day of relief and celebration."

In coming weeks, with more and more doses shipped out, "every parent who wants a vaccine will be able to get one," he said.

The Moderna vaccine, administered in two doses a month apart, will be available to children aged six months to five years in reduced doses of 25 micrograms (half the amount given children aged six to 11, and a quarter the dose for those 12 and older).

The Pfizer-BioNTech vaccine is now authorised for children aged six months to four years, and will be given in doses of three micrograms per injection – one-tenth the adult dosage.

The difference, however, is that children will receive three shots – the first two three weeks apart, followed by a third eight weeks later. (AFP)