(Updates headline)

Dec 8 (Reuters) - The U.S. health regulator has authorized COVID-19 shots from Moderna and Pfizer and its partner BioNTech that target both the original coronavirus and Omicron sub-variants for use in children as young as 6 months of age.

The amended authorization on Thursday from the Food and Drug Administration allows use of Moderna's bivalent shot as a booster in children 6 months through 5 years of age, two months after their initial vaccination.

Pfizer/BioNTech's updated shot can now be given as a third dose to those aged 6 months through 4 years, who have not completed their primary vaccination series or are yet to receive the third dose.

Children who have completed their initial three-dose vaccination with Pfizer's original shot are not yet eligible to receive the bivalent booster, the agency said.

The regulator added that data supporting use of Pfizer/BioNTech's bivalent shot as a booster in this age group is expected in January.

Shots for youngest children in the United States were only approved in June this year, making them the last group to become eligible for vaccination.

Government data shows that only 2.7% children under the age of two and less than 5% of children aged two to four years who are eligible have completed their primary vaccination series as of Nov. 30, representing a slow uptake of the initial vaccine doses in young children.

Moderna's vaccine for children under 6 is a two-dose, 25 microgram vaccine, with the shots given about four weeks apart. The Pfizer/BioNTech vaccine for the youngest children is a lower dose, 3-shot regimen given over at least 11 weeks.

Overall, 39.7 million people in United States have received a bivalent booster as of Nov. 30, data from the Centers for Disease Control and Prevention showed. (Reporting by Khushi Mandowara and Bhanvi Satija in Bengaluru; Editing by Sriraj Kalluvila)