Pfizer-BioNTech COVID-19 Vaccine vials setup for vaccinating children ages 5-11 at Katherine R. Smith Elementary School in San Jose, Calif., on Nov. 4, 2021. (Harika Maddala/ Bay City News)

The U.S. Food and Drug Administration on Thursday expanded its authorization for the omicron-specific COVID-19 booster vaccine to include children under age 5.

Children between the ages of 6 months and 5 years who received the initial Moderna COVID-19 vaccine will become eligible for the Moderna booster if they completed their initial vaccine series at least two months prior.

For children aged 4 and younger who have not yet started their three-dose primary series of the vaccine manufactured by Pfizer-BioNTech, the updated booster will now be used as the third dose following two doses of the original Pfizer vaccine, according to the FDA.

Children who have already received all three vaccine doses in the Pfizer series will not be eligible for a booster dose at this time, FDA officials said, but the agency plans to review safety and effectiveness data in January for giving those children a fourth dose with the updated booster.

“More children now have the opportunity to update their protection against COVID-19 with a bivalent COVID-19 vaccine, and we encourage parents and caregivers of those eligible to consider doing so — especially as we head into the holidays and winter months where more time will be spent indoors,” FDA Commissioner Dr. Robert Califf said in a statement.

The updated, “bivalent” boosters target both the initial COVID-19 strain and the BA.4 and BA.5 subvariants of the highly contagious omicron variant.

The updated boosters have been used for all booster vaccinations since the FDA approved them in early September, while the original mRNA vaccines are only currently authorized for initial vaccine series.

Anyone age 12 and up was originally eligible for the bivalent booster and children between the ages of 5 and 11 have been eligible since mid-October. Common side effects are similar to that of the original vaccine, including soreness at the injection site, fatigue and fever.

“Vaccines remain the best defense against the most devastating consequences of disease caused by the currently circulating omicron variant, such as hospitalization and death,” said Dr. Peter Marks, the director of the FDA’s Center for Biologics Evaluation and Research.

“Based on available data, the updated, bivalent vaccines are expected to provide increased protection against COVID-19,” Marks said.