Florida is among the states reporting the highest number of people opting for a booster shot, followed by Ohio, California, Illinois and Tennessee.
The estimated 1.1 million, included in an internal CDC briefing document reviewed by ABC News, likely is an undercount because although it counts Moderna and Pfizer shot recipients it ignores people who may have received the one-shot Johnson & Johnson vaccine and got another shot.
It's also unclear whether people who received a third shot did so under the direction of a doctor. The Food and Drug Administration hasn't authorized a third shot to boost immunity, although there are reports of some physicians encouraging severely immunocompromised patients to do so.
Boosters for the immunocompromised may be recommended by the FDA within weeks.