Pfizer Inc and its partner BioNTech SE said on Tuesday that they had submitted an application to the U.S. health regulator for the authorization of a booster dose of their COVID-19 vaccine for children aged 5 to 11 years.

The companies earlier this month reported data from a mid-to-late stage study showing a third dose of their shot increased protection against the original coronavirus version and the omicron variant among children in the age group.

It is unclear how much demand there is for a third vaccine dose in the age group. Just 28% of children aged 5 to 11 years - around 8.2 million - are fully vaccinated, according to data from the U.S. Centers for Disease Control and Prevention.

There has also been some skepticism on the need for boosters in younger children given the reduced risk of severe infection and hospitalization in the age group.

Pfizer and BioNTech have filed for the clearance of a 10-microgram booster dose for children 5 to 11 years. Adults receive a 30-microgram dose of the vaccine.

The primary two-dose COVID-19 shot from Pfizer and BioNTech was authorized in the United States for children 5 to 11 years in October.

The U.S. Food and Drug Administration (FDA) in January authorized the use of a third dose of the companies' COVID-19 vaccine for children ages 12 to 15. The agency has also authorized a booster shot for children aged 5 through 11 years who are immunocompromised.

Rival Moderna Inc is still waiting for a decision from U.S. regulators on the use of its primary COVID-19 series among age groups under 18 years.

It plans to submit an application to the FDA for the authorization of its COVID-19 vaccine among kids six months to five years by the end of April.