The Food and Drug Administration (FDA) on Thursday authorized an additional dose of the Pfizer-BioNTech or Moderna COVID-19 vaccines for immunocompromised Americans to better protect them as the highly contagious delta variant continues to rage.

On Friday, a Centers for Disease Control and Prevention (CDC) advisory committee voted overwhelmingly on Friday to recommend a booster dose for Americans with weakened immune systems, clearing the way for them to be administered nationwide.

Panel members voted 11-0 to issue the recommendation, which CDC director Dr. Rochelle Walensky later adopted, clearing the way for immunocompromised Americans to receive boosters.

"At a time when the Delta variant is surging, an additional vaccine dose for some people with weakened immune systems could help prevent serious and possibly life-threatening COVID-19 cases within this population," Dr. Walensky said in a statement.

"Emerging data suggest some people with moderately to severely compromised immune systems do not always build the same level of immunity compared to people who are not immunocompromised," Dr. Walensky said, adding: "While people who are immunocompromised make up about 3% of the U.S. adult population, they are especially vulnerable to COVID-19 because they are more at risk of serious, prolonged illness."

"CDC does not recommend additional doses or booster shots for any other population at this time," she noted.

The CDC, in a presentation, detailed that boosters should be administered to:

• People who haverecently undergone, or are currently undergoing, treatment for tumors or blood cancers
• People who have received organ transplants, or people who have recently received stem cell transplants
• People with advanced or untreated HIV
• People with conditions that weaken or damage the immune system
• People with chronic medical conditions, such as chronic kidney disease or asplenia, which weaken the immune system

"The country has entered yet another wave of the COVID-19 pandemic, and the FDA is especially cognizant that immunocompromised people are particularly at risk for severe disease," Dr. Janet Woodcock, acting commissioner of the FDA, said in a statement. "After a thorough review of the available data, the FDA determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna Vaccines."

The announcement applies to millions of Americans with compromised immune systems, specifically including "solid organ transplant recipients or those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise."

According to the CDC, about 7 million people, or 2.7% of U.S. adults, are immunocompromised.

The move by the U.S. follows that of several other countries, including France and Israel, which have similar recommendations.

It’s harder for vaccines to rev up an immune system suppressed by certain medications or diseases, so those patients don’t always get the same protection as otherwise healthy people — and small studies suggest for at least some, an extra dose may be the solution.

"Emerging data show that certain people who are immune compromised, such as people who have had an organ transplant and some cancer patients, may not have had an adequate immune response to just two doses of the Covid vaccine," Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention (CDC) said during a White House COVID-19 Response Team briefing on Thursday. "To be clear, this is a very small population. We estimate it to be less than 3% of adults."

"This action is about ensuring our most vulnerable ... are better protected against COVID-19," Dr. Walensky said.

Importantly, the decision only applies to this high-risk group, about 3% of U.S. adults. It’s not an opening for booster doses for the general population. Instead, health authorities consider the extra dose part of the initial COVID-19 vaccine prescription for the immune-compromised.

For example, France since April has encouraged that such patients get a third dose four weeks after their regular second shot. Israel and Germany also recently began recommending a third dose of two-dose vaccines.

However, in an interview with CBS News on Thursday, Dr. Anthony Fauci said that it is "likely" that all Americans will require booster shots against COVID-19 "in the future."

"It’s likely that that will happen at some time in the future,” Fauci said. “We’re already starting to see indications in some sectors about a diminution over time, that’s durability."

"We don't feel at this particular point that, apart from the immune compromised, we don't feel we need to give boosters right now," Dr. Fauci said, but noted that they are following data "in real time" and guidance could change.

Fauci said that "it is preferable" that people stick with the same brand when it comes to booster shots, and the FDA will provide guidance when the time comes.

"Inevitably there will be a time when we’ll have to get boosts," Dr. Fauci said on NBC's "TODAY Show" on Thursday, because "no vaccine, at least not within this category, is going to have an indefinite amount of protection."

One recent study of more than 650 transplant recipients found just over half harbored virus-fighting antibodies after two doses of the Pfizer or Moderna vaccines — although generally less than in otherwise healthy vaccinated people. Another study of people with rheumatoid arthritis and similar autoimmune diseases found only those who use particular medications have very poor vaccine responses.

There’s little data on how well a third dose works, and if it causes any safety problems such as an increased risk of organ rejection. Wednesday, Canadian researchers reported that transplant recipients were more likely to have high levels of antibodies if they got a third dose than those given a dummy shot for comparison. Other small studies have similarly found that some transplant recipients respond to a third dose while others still lack enough protection.

The FDA recommends that immunocompromised Americans better protect themselves by masking and practicing safe social distancing.

"In addition, close contacts of immunocompromised persons should get vaccinated, as appropriate for their health status, to provide increased protection to their loved ones," the FDA said in a statement.

CDC members were also asked about immunocompromised people who received the Johnson & Johnson vaccine; officials are currently investigating the matter, but noted that far fewer Americans have been inoculated with the single-shot jab, as opposed to the Moderna and Pfizer-BioNTech vaccines.

As of Thursday, Aug. 12, the U.S. has administered more than 198 million doses of Pfizer-BioNTech, more than 141 million doses of Moderna, and 13.78 million doses of Johnson & Johnson.

More than 89 million Americans have been fully vaccinated with the Pfizer-BioNTech immunization, and an additional 64 million have been fully vaccinated with Moderna's vaccine.

The Associated Press contributed to this report.