Pfizer Inc said on Tuesday that the U.S. Food and Drug Administration had expanded the emergency use authorization (EUA) of the company and its partner BioNTech SE's bivalent COVID-19 vaccine as a single booster dose in certain children.

The amended authorization is for children six months through four years of age who have completed their initial three-dose vaccination with Pfizer's original shot.

In December, the U.S. health regulator had authorized Pfizer/BioNTech's updated shot as a third dose to those aged six months through four years, who have not completed their primary vaccination series or are yet to receive the third dose.

The health regulator's amended authorization is based on data from 60 children, from the expanded age group, who completed primary vaccination with three doses and received a booster shot of Pfizer/BioNTech, and showed an immune response to both the original SARS-CoV-2 virus strain and to omicron BA.4/BA.5.

Shots for youngest children in the United States were only approved in June last 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.