Epidemiologist Dr. Anne Rimoin told CNBC that she agrees with White House chief medical advisor Dr. Anthony Fauci when he said that everybody will someday “likely” need a booster shot of the Covid-19 vaccines.

“Well, I think Dr. Fauci is right,” said Rimoin, a professor of epidemiology at the UCLA School of Public Health.

“What’s going to warrant a booster shot is, when we see real, waning effectiveness of this vaccine from keeping people from severe disease and from hospitalization or death. We’re not there yet, but when we are, that’s when we’re going to need a booster.”

The Centers for Disease Control and Prevention gave final approval Friday to start administering Covid-19 booster shots to Pfizer and Moderna vaccine recipients hours after a key panel unanimously voted to endorse third doses for immunocompromised Americans. The CDC’s decision followed the Food and Drug Administration’s approval late Thursday of the booster shots for immunocompromised patients. 

Rimoin told CNBC’s “The News with Shepard Smith” that both agencies made a “really important” decision when it comes to the immunocompromised population. 

“When they were able to get the vaccine, they didn’t really mount an immune response that was sufficient to protect themselves against this virus,” said Rimoin. “That’s why, right now, both the FDA and the CDC are recommending that these people get an additional dose, which has been shown in studies to improve immune response in about 1/3 to 1/2 of the population.” 

Immunocompromised patients make up roughly 2.7% of the U.S. adult population and 44% of hospitalized breakthrough infections, where someone gets infected even after they’ve been fully vaccinated.