The United States has already delivered more than 4 million doses of COVID-19 vaccines for children under 5 years old to sites that will administer the shots, a Department of Health and Human Services (HHS) spokesperson said on Thursday.

HHS has received about 4.3 million orders for vaccines for young children since pre-ordering began a few weeks ago, and will continue to take new orders, the spokesperson said.

"We made 10 million doses of vaccine available for ordering initially, with millions more available soon, so supply should not be a barrier to someone getting their young child vaccinated," the spokesperson told Reuters.

Delivery to vaccination sites such as pharmacies, doctors' offices and children's hospitals, began on Friday, immediately after the U.S. Food and Drug Administration granted the emergency use authorization.

The FDA authorized Moderna Inc's two-dose vaccine for children aged six months to five years and Pfizer-BioNTech's three-shot regimen for children aged six months to four years late last week.

There were nearly 20 U.S. million children below the age of 5 as of 2019, according to data from the U.S. Census Bureau.