By Jeff Wagner

UPDATE: Sources tell CBS News Monday night that U.S. health officials and experts are likely to announce the recommendation of boosters for all Americans eight months after receiving a second Pfizer or Moderna COVID-19 vaccine shot. Pending authorization by the FDA, shots may start being administered as early as mid-September.

MINNEAPOLIS (WCCO) — A small population of Americans can now get a third dose of the COVID-19 vaccine. It’s a very specific group, and the type of vaccine people initially received plays a role.

READ MORE: Alliance Seeks To Preserve Duluth's Most Endangered Buildings

Who is eligible to get a third vaccine dose? And why does that group need it? Good Question.

As the Delta variant quickly spreads across the United States, doctors in hospitals crowded with COVID-19 patients wonder if more can be done to protect people.

“It’s making us even come back and question, should we be giving booster doses now to those with underlying medical conditions?” said Dr. Teri Oakes Dyess, Director of Hospital Medicine at Saint Dominic Jackson Memorial Hospital in Mississippi.

Last week, the CDC approved the use of a third vaccine dose for specific groups. While it might sound like a booster shot, Kris Ehresmann, infectious disease director with the Minnesota Department of Health, says it’s not.

“A booster is a dose of vaccine that will be given to people who got vaccinated, had a strong immune response, but over time there may be waning immunity,” she said.

What the CDC approved recently is simply a third dose, of either Pfizer or Moderna, because the first two doses were not effective enough for certain immunocompromised people.

READ MORE: Sen. Omar Fateh Says Charter Amendment Vote Gives Mpls. Chance For 'New Approach To Public Safety'

“By adding a third dose, the effectiveness (of the vaccine) moved from about 40% to 60%,” Ehresmann said.

The eligibility list for a third dose represents just 2.7% of the population. It includes people who have been receiving cancer treatment, organ or stem cell transplant recipients, moderate or severe primary immunodeficiency, advanced or untreated HIV, and active treatment of drugs that suppress an immune response.

Those eligible for a third dose should get the same brand as their initial doses, Pfizer or Moderna, however, experts say it is acceptable to switch between the two if necessary.

What is someone to do if they’re eligible for a third dose but they got the Johnson & Johnson vaccine?

“At the federal level, there’s insufficient data to expand the emergency use authorization for the Johnson & Johnson vaccine,” said Ehresmann.

That means those people have to wait, with no timetable on when a second Johnson & Johnson dose could be approved. They should not substitute Johnson & Johnson for one of the mRNA vaccines (Pfizer/Moderna).

What about people with underlying conditions like asthma? Ehresman said if your condition isn’t on the eligibility list, that means the data did not show a third dose was necessary.

MORE NEWS: 'It Was Pretty Chaotic': 3 Dead In Montana Amtrak Train Derailment

For more information from the CDC on getting a third dose, click here.

Jeff Wagner