The U.S. Food and Drug Administration authorized Pfizer booster shots Wednesday for seniors and other high-risk Americans, paving the way for third doses to be offered as early as the end of the week.

The decision would allow for anyone over the age of 65 to get a booster, as well as people as young as 18 if they have a medical condition that puts them at risk of severe COVID-19 or if they work a frontline job that makes it more likely that they would get infected.

FDA's acting commissioner, Dr. Janet Woodcock, said that list should include health care workers, teachers and grocery story workers, as well as people in prisons and homeless shelters.

"This pandemic is dynamic and evolving, with new data about vaccine safety and effectiveness becoming available every day," Woodcock said in a statement. "As we learn more about the safety and effectiveness of COVID-19 vaccines, including the use of a booster dose, we will continue to evaluate the rapidly changing science and keep the public informed."

The deliberation follows a recommendation last week by the FDA's independent scientific advisers that, while protection from vaccination is strong, immunity probably wanes after six months and is important to replenish for certain high-risk groups.

The advisory panel, called the Vaccines and Related Biological Products Advisory Committee, said last Friday there wasn't enough evidence yet to recommend every vaccinated adult get a third dose.

Instead, the panel recommended the extra shot for those 65 and older or at high risk of severe COVID-19.