Pfizer and BioNTech on Thursday announced positive results from a clinical trial on the safety and immune response of a third dose of their Covid vaccine in children aged five through 11, adding they would soon seek regulatory authorization.

Third doses of the vaccine are recommended for those aged 12 and up, and a fourth dose was recently recommended for people over 50. 

Younger children -- except for those with immune compromising conditions -- have not been eligible for the third, making them more susceptible to infection from Omicron and its BA.2 subvariant.

BA.2 is now the globally dominant strain, and is behind a current spike in cases in the northeastern United States.

In the phase 2/3 trial, the companies analyzed data from 140 children aged five through 11, approximately six months after the second dose.

The dosage in this group is 10 micrograms, which was selected for safety reasons as children are more susceptible to side effects. The dose for those 12 and up is 30 micrograms.

Across the 140 children analyzed, the third dose was well tolerated, revealing no new safety concerns.

They also analyzed blood sera from a group of 30 individuals, finding that a third dose caused a 36-fold increase in levels of infection-blocking neutralizing antibodies against Omicron, compared to two doses.

Pfizer and BioNTech plan to soon submit the data to the US Food and Drug Administration (FDA), the European Medicines Agency and other regulatory agencies.

Most countries, including the United States, haven't yet authorized Covid vaccines for infants and very young children.

Last month, Moderna said it was pursuing approval for its vaccine in children aged six months through five years, using a two-dose regimen.

Pfizer's vaccine for this group was meant to be considered by the FDA in February but the agency postponed the meeting, because it wanted more data on how it would perform with three doses.