Pfizer Inc and its German partner BioNTech SE have applied for emergency use authorization of their omicron-adapted COVID-19 vaccine in the United States as a booster dose for children aged six months through four years, the companies said on Wednesday.

The omicron-adapted vaccine is currently authorized by the U.S. health regulator as the third dose of the three-dose primary course of vaccination in the country for children in this age group.

If the Food and Drug Administration (FDA) authorizes the vaccine as a booster dose, children who have completed their primary series - either with three doses of the Pfizer-BioNTech's original vaccine or with two doses of their original and one dose of the adapted vaccine - would be eligible to receive the booster dose at least two months after the completion of their primary series.

The companies plan to also submit applications to the European Medicines Agency (EMA) and other global regulatory authorities to extend the marketing authorization of the Omicron-adapted vaccine to include its use in children under five years as both the primary course of vaccination and booster dose.

Last week, the companies also filed an application to the FDA for a full approval of their omicron-adapted COVID vaccine as a primary course and a booster dose for individuals 12 years of age and above.