U.S. federal regulators authorized booster shots of the Pfizer (PFE  )-BioNTech (BNTX  ) COVID-19 vaccine for children as young as 5-years-old.

Centers for Disease Control and Prevention (CDC) Director Dr. Rochelle Walensky signed off on the third dose on Thursday after the agency's Advisory Committee on Immunization Practices voted in favor of recommending a third dose to children ages 5 to 11 at least five months after their primary two-shot series.

According to CDC data, less than 30% of the 28 million American children in this age range have received two doses of a COVID vaccine.

"Vaccination with a primary series among this age group has lagged behind other ages groups leaving them vulnerable to serious illness," said Walenksy in a press statement. "With over 18 million doses administered in this age group, we know that these vaccine are safe, and we must continue to increase the number of children who are protected. I encourage parents to keep their children up to date with CDC's COVID-19 vaccine recommendations."

The CDC's decision follows the U.S. Food and Drug Administration's (FDA) extended Pfizer's emergency use authorization to include booster shots for this age group.

Dr. Peter Marks, director of the agency's Center for Biologics Evaluation and Research, said in a statement that data continues to demonstrate that protection offered by current COVID vaccines wanes over time and a third dose can boost protection for younger children as it has in older age groups.

The regulators' decisions came after Pfizer data from an ongoing clinical trial showed virus-neutralizing antibodies against both the Omicron variant and the original strain of the coronavirus increased one month after receiving a booster dose. Regulators also did not identify any new safety concerns and booster shots only elicited mild side effects in the children participating in the study.

"While it has largely been the case that COVID-19 tends to be less severe in children than adults, the Omicron wave has seen more kids getting sick with the disease and being hospitalized, and children may also experience longer term effects, even following initially mild disease," said FDA Commissioner Dr. Robert Califf in a press statement Tuesday. "Vaccination continues to be the most effective way to prevent COVID-19 and its severe consequences, and it is safe."

Almost every age group in the U.S. is now eligible to receive at least three doses of COVID vaccine, only expecting children under the age of 5. These youngest Americans are still eligible for a primary dose series, with Pfizer and Moderna (MRNA  ) both previously submitting authorization request forms to the FDA. The FDA's vaccine advisory committee is set to meet next month to review those requests.