The Centers for Disease Control and Prevention has approved giving third vaccine doses to people with compromised immune systems - but, they're not the booster shots that health officials have been discussing.

CVS and Walgreens pharmacies began rolling out third vaccine doses to people with weakened immune systems Saturday morning.

About 9 million people in the U.S. are considered immunocompromised due to an underlying condition or medication they are taking.

Among those eligible for a third dose are people who've had a transplant, received cancer treatment or have HIV.

John Gast, of Roslyn Heights, says he recently recovered from cancer and plans to get a third dose.

The third dose is completely different from a COVID-19 booster shot, which has not been approved by the Food and Drug Administration or the CDC.

Doctors say the difference between a third dose and booster shot is in the chemical makeup.

"A third dose is going to be identical to the first and second doses. Boosters do not have be identical. It depends on what the data shows in terms of our response to a vaccine. So they can be chemically different from the prior doses," says Northwell Health vaccine program director Dr. Matt Harris.

A booster acts as a supplement for those who have already mounted an immune response from the vaccine's initial two-shot sequence.

The CDC says immunity from some vaccines can wear off over time, and at that point, a booster dose is needed to bring immunity levels back up.

Dr. Anthony Fauci says tests are being evaluated on a weekly basis to see when a booster shot will be needed for the general public.