The Food and Drug Administration approved Covid-19 vaccine booster shots for people with compromised immune systems, a highly anticipated move designed to protect some of the most at-risk Americans from the highly contagious Delta variant.

“Today’s move enables doctors to boost immunity in certain immunocompromised individuals who need additional protection from COVID-19,” said Acting FDA Commissioner Dr. Janet Woodcock in a statement.

The agency said organ transplant recipients and people with similar immunodeficiency can receive a third dose of the Pfizer BioNTech or Moderna vaccines. The FDA also stressed that other fully vaccinated individuals are “adequately protected” and do not currently require an additional dose.

The FDA’s OK is not the final go-ahead. The Centers for Disease Control and Prevention’s vaccine advisory committee scheduled a meeting on Friday to consider vaccinations for immunocompromised Americans. If they make a recommendation and the CDC approves it, third shots could begin immediately.

The Senior Medical Advisor to the White House, Dr. Anthony Fauci said last week that federal health officials would accelerate regulatory efforts to clear third doses for those people, including cancer and HIV patients or those who have had organ transplants. He said new data suggested they fail to generate an adequate immune response after two doses of a Covid vaccine.

“Immunocompromised people are vulnerable,” said Fauci on Thursday. “It is extremely important for us to give these people their boosters and we are working on it now and we will make sure that this is done as soon as possible. … It is a very high priority.”

Such people make up only about 2.7% of the U.S. adult population but account for about 44% of hospitalized breakthrough Covid cases in which a fully vaccinated person becomes infected, according to recent data from an advisory group from the Centers for Disease Control and Prevention .

Studies suggest that a third shot might help people whose immune systems don’t respond as well to a first or second dose.

Four small studies cited by the CDC last month showed that 16% to 80% of people with compromised immune systems had no detectable antibodies to Covid after two shots. Among vaccinated patients who had no detectable antibody response, 33 to 50% developed an antibody response after receiving an additional dose, according to the CDC.

Given the rising delta in the US, doctors fear that leaving such groups unprotected against the virus could lead to even more dangerous variants.

Fauci, who spoke to CBS This Morning on Thursday, said it was “likely” that everyone will need a booster dose at some point. But the priority at the moment is to give boosters to people with weakened immune systems.

Some doctors have long pushed for the US to allow extra doses for immunocompromised populations, and many of them are already finding extra doses of the vaccines themselves, medical experts say.

A CDC advisory group met last month to see if fully vaccinated Americans with compromised immune systems needed a booster dose after data showed they were less likely to have antibodies to fight the disease and more likely to have a breakthrough infection.

“The most difficult to vaccinate are people with immunosuppression,” said Dr. Dan Barouch, an immunologist at Harvard Medical School. Data suggests that at least a third of them respond well when given a third dose, he added.

The FDA’s move came after the World Health Organization last week urged wealthy nations to temporarily stop distributing booster vaccines, highlighting the injustice in vaccines around the world. However, World Health officials stressed that they were not referring to the extra doses that might now be required for certain groups, such as people with compromised immune systems.

Other countries such as France are already giving third vaccinations to people with cancer or other immune deficiencies. Israel last month announced plans to offer booster syringes to people over 60, another high-risk group, as the syringe seems to be less effective in these people.

In the US, Mississippi, a state with some of the lowest vaccination rates, advises doctors to consider a booster dose for people with compromised immune systems. The local health authorities recommend waiting at least four weeks after the full vaccination before receiving a booster vaccination.

This is a developing story. Please check again for updates.

– CNBC’s Christine Wang contributed to this report.