CHICAGO (NEXSTAR) — Dr. Anthony Fauci, America’s top infectious disease expert, said he believes booster shots will be available for Americans starting Sept. 20 – but not everyone will be able to get a dose right away.

First off, booster shots still haven’t been approved for use by the Food and Drug Administration. The FDA is set to consider them Sept. 17, but until that happens – no boosters for anyone who isn’t immunocompromised. (The Centers for Disease Control already recommends those who are moderate to severely immunocompromised talk to their doctors about getting a third dose.)

If the FDA does give boosters the green light by the Sept. 20 target date, there’s another hitch: It’ll probably just be for one type of vaccine, Pfizer.

That’s because the FDA and CDC say they haven’t gotten enough data from Moderna to authorize a third dose yet. The agencies have more data on Pfizer’s use from Israel, so they feel more comfortable giving a third dose the go-ahead.

Data for boosters on Johnson & Johnson’s single-dose vaccine won’t be available for months, since that shot wasn’t approved until late February, officials said.

Since the CDC isn’t recommending mixing and matching vaccines yet, that means anyone who received Moderna or Johnson & Johnson will have to wait. Mixing vaccine types is likely safe, said the World Health Organization’s Dr. Kate O’Brien, but health leaders want to see more clinical trials before approving it.

But even if you did get Pfizer for your first two doses, you still might not be able to get that third shot just yet. When the CDC announced the plan to start rolling out booster doses, it said they’d be available for all Americans if it’s been eight months since your second dose.

So that means you’ll only be eligible for a booster shot on Sept. 20 if:

• The FDA approves their use in time
• You received Pfizer for your first two doses
• The date of your second dose was on or before Jan. 20, 2021

Because health care workers, nursing home residents and elderly people were the first populations to be prioritized for the vaccine, they’ll also become the populations eligible for the boosters the soonest.

However, some folks haven’t been waiting for FDA approval or their eight-month benchmark. The CDC estimates upwards of 1.1 million people have gotten an unauthorized third shot already, according to documents reviewed by ABC News.

The Associated Press contributed to this report.