The FDA is expected to approve the very first COVID-19 booster shots.

But the booster shots won't initially be for everyone. A source familiar with the process says the agency is expected to greenlight booster shots for the immunocompromised before the end of the week.

"An additional dose could help increase protections of these individuals, which is especially important as the delta variant spreads," says CDC Director Dr. Rochelle Walensky.

Over 9 million Americans are immunocompromised, and there are concerns over the vaccines' effectiveness over time. A Johns Hopkins study says that the people with underlying conditions are 485 times more likely to end up in the hospital or die from COVID - even if they are vaccinated.

"There are some patients that don't respond at all where you don't see any antibodies, where when they receive that third dose you start to see additional antibodies," says Dr. Onisis Stefas, chief of pharmacy at Northwell Health. "So, they could receive anywhere between 30-50% response to the vaccine, where getting that third dose now results in higher response and better protection against the virus.

People with compromised immune systems include transplant patients who take immune-suppressing drugs to prevent rejection of new organs and others who are undergoing cancer treatment and people with HIV.

John Gast, of Roslyn Heights, says if a booster is needed, he is all for it.

"I have diabetes. I have a heart condition and just recently recovered from cancer. So, it's very important to me," says Gast.

If approved, the boosters would apply to those who got the Pfizer and Moderna vaccines - which are two doses each. More studies need to be done for the Johnson & Johnson vaccine. This would also be the first authorization of an additional dose in the U.S.