WASHINGTON, Dec 29 (Reuters) - Approval of a third COVID-19 vaccine dose for U.S. children aged 12-15 could be days or weeks away, the head of the Centers for Disease Control and Prevention (CDC) told CNN in an interview on Wednesday. Asked when children in that age group could receive a booster shot, CDC Director Rochelle Walensky said: "So the FDA (Food and Drug Administration) is looking at that right now. Of course, the CDC will swiftly follow as soon as we hear from them, and I"m hoping to have that in the days to weeks ahead." It was not immediately clear, however, that Pfizer Inc (PFE.N) and its partner, BioNTech SE (22UAy.DE) -- the only manufacturer whose COVID-19 vaccine is approved for kids -- has formally sought FDA approval for a booster in children ages 12-15. Representatives for Pfizer and the FDA did not immediately respond to a request for comment. Third booster shots are approved for U.S. children ages 16 and 17 and all U.S. adults. read more Questions over boosters for some older children come as the United States faces a new wave of infections from the Omicron variant as cases reach a record high. read more While U.S. officials have said vaccinations protect against serious infection and death with Omicron, some children ages 12-15 are now more than six months out from their earlier inoculations. U.S. children ages 5-12 were recently cleared for the vaccine so have just begun their inoculations in recent weeks. No vaccine is approved for children under age 5, something Walensky said she did not expect anytime soon. "The companies and manufacturers are working toward data for children under 5. That will not be in the month ahead," she told CNN. Pfizer earlier this month said it was looking at a three-dose regimen of its COVID-19 vaccine for children ages 2 to 16 while results showed three doses among people older than 16 offered greater protection. read more