The Food and Drug Administration is poised to authorize a third COVID shot for the immunocompromised on Thursday, sources told ABC News.

About 3% of the population would qualify, Dr. Anthony Fauci told ABC News' "Good Morning America."

He said the boosters would be "for example, people who have transplantation and are on immunosuppressive drugs for that; people on therapy for cancer -- cancer chemotherapy; people with advanced HIV disease; and people who are receiving immune suppressive therapy for a variety of diseases."

When asked if the boosters would be available to everyone, Fauci said, "You have to follow people, which we're doing in real-time, mainly a non-immune compromised, either an elderly person or a younger person … to determine if their level of protection goes below a critical level."

He added, "If and when it does, and it's likely that it will because no vaccine is gonna last forever, we're gonna be ready and have a plan to give those individuals the additional dose they might need."

The University of Mississippi Medical Center, overwhelmed with COVID-19 patients due to the delta variant, is opening a field hospital in one of the center's garages.

The unit will have 50 beds and will likely be available to take in patients by Friday, Gov. Tate Reeves wrote on Twitter Wednesday.

The news comes as Mississippi recorded 3,163 positive COVID-19 cases on Wednesday, according to data from Johns Hopkins University.

Four school districts in Georgia recently paused in-person learning as positive cases of the coronavirus among staff and students swelled in the first days of school this month.

The districts -- Macon, Taliaferro, Glascock and Talbot -- account together for less than 1% of Georgia's 1.7 million students, but the need to shut down in-person learning so early in the school year worries district officials.

"The difference now in this outbreak that we see than the outbreak that happened last school year is that this seems to be more centered on kids rather than adults, so that scares me to death," Jack Catrett, the superintendent of schools in Talbot County, told Columbus ABC affiliate WTVM.

Talbot County, which had 11 students test positive on Friday, shut its doors to students for one week, with kids returning Monday. The other three districts have planned for two-week pauses to in-person learning.

-ABC News' Will McDuffie

Dr. Anthony Fauci, the nation’s top infectious disease expert, said in an interview with ABC News anchor Linsey Davis Thursday that it's difficult to know whether the U.S. might see a sudden drop-off in cases, but that the best way out is by ramping up vaccinations.

"The easiest way to turn this thing around is to get as many people vaccinated as you possibly can," he said.

"Wouldn't you rather get the drop-off due to the fact that you're preventing infections rather than you've gotten so many people infected?" he later said.

Fauci said he is most worried about those who are unvaccinated, and that it was "painful" to hear of people regretting their decision not to get vaccinated and then getting ill. 

"What's absolutely certain is that the people who are vulnerable to getting severe disease, hospitalization and death is overwhelmingly the unvaccinated," he told Davis, noting that the latest data shows that 99% of people who have died of COVID-19 are unvaccinated. "That should strongly urge you to get a shot."

-ABC News' Jon Schlosberg, Anne Flaherty and Brendan Rand