Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, listens during a Senate Fund Subcommittee hearing in Washington, DC, on Jan.

Stefani Reynolds | Swimming pool | Reuters

Federal health officials are working “ASAP” to get a third Covid-19 vaccine approved for Americans with weakened immune systems, said Dr. White House chief physician Anthony Fauci on Thursday.

It is now clear that immunocompromised populations – which include patients with cancer, HIV, or organ transplants – generally fail to generate an adequate immune response after two doses of a Covid vaccine, Fauci said.

“Immunocompromised people are vulnerable,” said Fauci during a Covid briefing at the White House. “It is extremely important for us to give these people their boosters and we are working on it now and we will do this as soon as possible. … It is a very high priority.”

Immunocompromised populations make up only about 2.7% of the adult US population. Still, they account for about 44% of hospitalized breakthrough Covid cases – an infection in a fully vaccinated person, according to data released late last month by 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 immunocompromised patients who had no detectable antibody response, 33 to 50% developed an antibody response after receiving an additional dose, according to the CDC.

“From the observational data we have made, it now appears that they are generally not giving an adequate response that we believe would be adequately protected,” Fauci said Thursday.

Other countries such as France are already giving third vaccinations to people with cancer or other immune deficiencies. Israel announced last month that it would offer booster syringes to people over the age of 60 as the syringe seems to be becoming less effective in these people.

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

Dr. Scott Gottlieb, who headed the Food and Drug Administration during Donald Trump’s presidency from 2017 to 2019, told CNBC on Monday that he believes the elderly and immunocompromised people will receive booster shots by September or October.