Roughly 22 million Americans are at least six months past their second dose of the Pfizer-BioNTech vaccine, according to the Centers for Disease Control and Prevention. About half of them are 65 and older.

Millions of Americans who received the Moderna and Johnson & Johnson vaccines are still waiting to learn whether they, too, can get boosters. The F.D.A. is expected to take up the question of boosters for them in short order.

The F.D.A.’s decision will be followed as soon as Thursday by a recommendation from the C.D.C., which issues guidance on vaccine policy for clinicians and public health officials throughout the United States. An advisory committee of the C.D.C. is now in the midst of a two-day meeting on the issue. But even if the C.D.C. takes a different stance, health care providers are now authorized to offer third shots to Pfizer-BioNTech recipients who meet the F.D.A.’s eligibility criteria.