The companies plan to file for emergency-use authorization in the U.S. in the coming days based on the results, according to a statement Thursday. They will also submit the data to other regulators around the world.

In the trial, 140 kids ages 5 to 11 were given a third shot of a 10 microgram dose of the vaccine about six months after their second dose. A month later, antibodies against the original COVID virus were roughly sixfold higher compared to what they had been a month after the second dose. 

The researchers assessed neutralizing antibodies against Omicron in 30 of the children who got the booster, and those levels increased 36-fold after the third dose, the companies said. 

Pfizer shares rose 0.9% at 9:55 a.m. in New York while BioNTech’s American depositary receipts gained 0.1%.

The third shot was well tolerated and no new safety signals were observed, the companies said.

The data “reinforce the potential function of a third dose of the vaccine in maintaining high levels of protection against the virus in this age group,” the two companies said in a statement.

It’s not clear how big the uptake would be if the extra dose for children is authorized by the Food and Drug Administration. In the U.S., only 28% of children ages 5 through 11 are fully vaccinated, according to the Centers for Disease Control and Prevention, and the pace of vaccination in the age group has declined significantly since the peak of the Omicron wave in early January.

—With assistance from Riley Griffin.