The U.S. Food and Drug Administration also signed off on "mixing and matching" booster shots, so adults do not have to receive an additional dose from the same brand that first vaccinated them.
Those eligible for the booster shots include all seniors 65 and older, adults who are at "high risk of severe Covid-19" and adults with frequent exposure to the virus because of where they live or work.
Acting FDA Commissioner Janet Woodcock said Wednesday in a statement that "vaccination continues to be the safest and most effective way to prevent Covid-19, including the most serious consequences of the disease, such as hospitalization and death."
"The available data suggest waning immunity in some populations who are fully vaccinated," she added. "The availability of these authorized boosters is important for continued protection against COVID-19 disease.”
The Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices is scheduled to discuss recommendations for use of the additional doses on Thursday.
More than 9 million Americans have already received booster shots from Pfizer, the CDC said, while roughly 1.7 million have received a third dose of Moderna's vaccine.
Earlier this month, the FDA's Vaccines and Related Biological Products Advisory Committee voted 19-0 to back Moderna's booster. The panel also gave the green light to Johnson & Johnson's booster.
The White House, which is expecting regulators to make vaccines available in the coming weeks to 5-to-11-year-olds, on Wednesday laid out plans to ensure that pediatric and primary-care offices, pharmacies, and school and rural health clinics will be ready to administer shots if the vaccine receives federal authorization.