COVID vaccine makers Pfizer and Moderna received U.S. regulatory clearances for their omicron-targeted boosters for the youngest children.

Moderna's vaccine was authorized for use at least two months after completion of the primary series in children 6 months through 5 years of age, according to a statement Thursday from the U.S. Food and Drug Administration.

The Pfizer shot can be given to children 6 months through 4 years of age who have not yet begun their three-dose primary series or have not yet received the third dose of their primary series.

Children 6 months through 4 years of age who have already completed their three-dose primary series with the Pfizer shots aren't eligible for an omicron booster dose, the statement said. Data in support of giving an updated bivalent booster dose for this group are expected to be submitted to the agency in January, and that data will be reviewed as quickly as possible, the statement said. Even so, children in this age group who have completed their primary series are still be expected to have protection against the most serious outcomes from the currently circulating omicron variant, the regulator added.

Updated booster shots have already been authorized for use in children and adults from the age of 6 and up.

FDA Commissioner Robert Califf encouraged parents and caregivers to consider giving children extra protection especially with the approach of the holidays and winter months, when more time will be spent indoors.

"As this virus has changed and immunity from previous COVID-19 vaccination wanes, the more people who keep up to date on COVID-19 vaccinations, the more benefit there will be for individuals, families and public health by helping prevent severe illnesses, hospitalizations, and deaths," Califf said in a statement.

Last month the Biden administration announced a six-week advertising campaign to convince people to get the shots in response to a slow uptake of the updated shots. Bivalent boosters are needed because immunity wanes over time and variants continue to come out every several months, Anthony Fauci, outgoing director of the National Institute of Allergy and Infectious Diseases, said in a White House briefing at the time.