The Food and Drug Administration is planning to authorize a third shot for the immune-compromised on Thursday, two sources familiar with the plans confirmed to ABC News.
About 2.7% of U.S. adults are considered immunocompromised.
Asked to comment on the plans, the FDA said its "closely monitoring data as it becomes available from studies administering an additional dose of the authorized COVID-19 vaccines to immunocompromised individuals."
"The agency, along with the CDC, is evaluating potential options on this issue, and will share information in the near future," the FDA said in a statement.
At a July meeting, members of ACIP were largely supportive of giving immunocompromised people a third dose to boost their immunity and they called on the FDA to move on the issue.
Data presented at the meeting indicated a booster shot could increase antibodies in an immunocompromised person by up to 50%.
Dr. Sara Oliver, an epidemiologist with the CDC who presented the findings, also explained how immunocompromised people are a priority group for booster research because they're at greater risk of serious COVID-related consequences.
For example, in one U.S. study, 44% of hospitalized breakthrough cases were immunocompromised people. An Israeli study found it was around 40%. Breakthrough cases, which are expected, refer to people who test positive for COVID-19 while fully vaccinated. The vaccines are highly effective against severe disease and hospitalization, but it's possible for people to develop mild or asymptomatic illness even when vaccinated.
Pressure continued to mount as other countries, including France and Israel, approved boosters for those who are immunosuppressed. Without official action, patients have also taken matters into their own hands.
The CDC estimated that more than 1 million people who have received the Moderna or Pfizer vaccine have gone back for an unauthorized booster shot.
The estimated 1.1 million, included in an internal CDC briefing document reviewed by ABC News, likely is an undercount because although it counts Moderna and Pfizer shot recipients, it ignores people who may have received the one-shot Johnson & Johnson vaccine but got another shot.
It’s also unclear whether people who received a third shot did so under the direction of a doctor. Florida is among the states reporting the highest number of people opting for a booster shot, followed by Ohio, California, Illinois and Tennessee.
For Phil Canuto, who was left immunocompromised after a kidney transplant 19 years ago, the news of boosters on the horizon left him elated.
Canuto, who is from Akron, Ohio, had no immune response after two doses of the Pfizer vaccine. The increasing spread of the delta variant and data showing that even vaccinated people can spread the virus has put him and his family members on high alert.
"I know a lot of transplant patients have been trying to get extra shots surreptitiously, lying to pop-up vaccination sites about their previous shots," he said, adding that this news will give patients the chance to make medically informed decisions with their doctors.
In an interview Wednesday with ABC News' Linsey Davis, Dr. Anthony Fauci, chief White House medical adviser, said he doesn't support just anyone getting a booster shot.
"What I support is people following the guidelines that will be coming out for those who are immunocompromised, as well as those who are now being followed in cohorts to be determined," he said, "just what the appropriate time to give to a booster."
Fauci said approving boosters for immunocompromised Americans was a "very high priority" at a White House briefing in early August.
"It is extremely important for us to move to get those individuals their boosters. And we are now working on that, and we'll make that be implemented as quickly as possible. Because for us and for the individuals involved, it is a very high priority," Fauci said.
Fauci also told governors privately on a National Governor's Association call that immunocompromised people will be first up in the queue for boosters, and that the FDA was "working on a mechanism" to get additional doses to that group "as soon as we can," according to a readout of the call obtained by ABC News.
He also said he expected the general population to eventually follow.
"I think eventually we all will go that direction," Fauci said, without specifying when.
ABC News' Anne Flaherty contributed to this report.