The Food and Drug Administration on Thursday expanded its emergency use authorization for Pfizer-BioNTech’s COVID-19 booster shots to include 16- and 17-year-olds, making them the first available for that age group in the U.S.

Hours later, the Centers for Disease Control and Prevention (CDC) expanded its recommendation to the age group, clearing the way for booster shots to begin for millions of teens.

"Today, CDC is strengthening its booster recommendations and encouraging everyone 16 and older to receive a booster shot," CDC Director Dr. Rochelle Walensky said in a statement. "Although we don’t have all the answers on the Omicron variant, initial data suggests that COVID-19 boosters help broaden and strengthen the protection against Omicron and other variants."

"We know that COVID-19 vaccines are safe and effective, and I strongly encourage adolescents ages 16 and 17 to get their booster if they are at least 6 months post their initial Pfizer vaccination series," Dr. Walensky added.

Sixteen- and 17-year-olds are now eligible for the booster shot six months after receiving their second dose of the Pfizer vaccine. The Pfizer-BioNTech vaccine has been the only one authorized for people under 18 years old.

“Since we first authorized the vaccine, new evidence indicates that vaccine effectiveness against COVID-19 is waning after the second dose of the vaccine for all adults and for those in the 16- and 17-year-old age group,” Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in a news release. “A single booster dose of the vaccine for those vaccinated at least six months prior will help provide continued protection against COVID-19 in this and older age groups.”

Around 2.6 million 16- and 17-year-olds were fully vaccinated as of six months ago. To date, more than 4.7 million adolescents in that age group, or 56%, are fully inoculated.

Pfizer Chairman and CEO Albert Bourla called the FDA’s decision “a critical milestone as we continue to stay vigilant in addressing the virus.”

“From the beginning, we aimed to provide strong, safe protection to as many people as possible in an effort to end this pandemic,” he said in a news release. “While new variants, including Omicron, emerge across the globe, we believe that the best way to minimize the spread of COVID-19 and any future variants is getting all eligible people fully vaccinated with the first two-dose series and a booster dose as recommended.”

On Wednesday, Pfizer acknowledged that two shots of its vaccine might not provide significant protection against the omicron variant but that early lab results show that a third dose appears to increase antibody protection 25-fold compared to two shots. Still, health officials stress that the delta variant accounts more than 99% of cases in the U.S.

The Pfizer vaccine received emergency use authorization for people 16 and older in December 2020. The shot then received full FDA approval in August.

The FDA and CDC granted emergency use authorization to booster shots for most adults in September and then expanded it last month to cover everyone 18 and older.

The U.S. is currently experiencing another surge in COVID-19 infections. The seven-day average for new daily cases as of Tuesday was 117,488, up 83% from Oct. 24. Deaths are up 51% since Nov. 27, to nearly 1,100 a day.