The F.D.A. authorized a third dose of Covid vaccines for immunocompromised people.

The Food and Drug Administration on Thursday authorized third doses of Pfizer-BioNTech’s and Moderna’s coronavirus vaccines for some people with weakened immune systems, giving physicians more leeway to protect those who did not respond enough to an initial series of shots.

The authorization, in the form of updates to the existing emergency use authorizations for the two vaccines, applies to people who received solid organ transplants and others with similarly compromised immune systems, the F.D.A. said.

The agency’s decision came a day before the Centers for Disease Control and Prevention’s independent advisory committee was set to consider and vote on whether to recommend the move. The committee is likely to give its approval, and the C.D.C. would follow with its own endorsement of the additional doses.

“The F.D.A. is especially cognizant that immunocompromised people are particularly at risk for severe disease,” Dr. Janet Woodcock, the acting F.D.A. commissioner, said in a statement. “After a thorough review of the available data, the F.D.A. determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna Vaccines.”

The authorization of the third doses kicks off what promises to be a busy next stretch for federal vaccine regulators — and a new phase of the nation’s inoculation drive. By the start of next month, the agency is expected to grant full approval to Pfizer-BioNTech’s vaccine. That will most likely prompt a wave of vaccination mandates from companies and organizations that waited to require vaccination until the F.D.A. fully cleared a vaccine.

At the same time, government scientists and regulators are grappling with whether more Americans will need booster shots, a hotly debated move that many scientists argue is not yet supported by data. Other countries such as Israel and Germany have implemented booster policies.

“Other individuals who are fully vaccinated are adequately protected and do not need an additional dose of Covid-19 vaccine at this time,” Dr. Woodcock said in her statement Thursday, adding that the agency was “actively engaged in a science-based, rigorous process with our federal partners to consider whether an additional dose may be needed in the future.”

The United States is the latest country to begin offering third doses to those with weaker immune systems. France has offered additional vaccine doses to certain people with poor immune responses since April, and Germany and Hungary recently followed suit.

About 3 percent of Americans have weakened immune systems for a variety of reasons, from a history of cancer to the use of certain medications such as steroids.

The F.D.A.’s decision to limit the category of people with weakened immune systems who should receive the extra dose was expected. Many scientists argue that the immunocompromised population is too diverse to uniformly recommend additional shots of coronavirus vaccine. Some may be protected by the standard vaccine dosage, despite their conditions. Others may be poorly shielded by the vaccines, but unable to benefit from an additional shot.

Studies suggest that patients such as organ transplant recipients are in between — often showing little immune response to the standard vaccine regimen, but benefiting from a third shot. One recent randomized, placebo-controlled study by Canadian researchers found that a third dose of the Moderna vaccine improved the immune response of people in that group.