When asked by a reporter on Wednesday about why fully vaccinated individuals aren't getting as many boosters, Fauci noted that "about half" of all eligible Americans have received booster doses so far. Fauci made the remarks alongside COVID-19 response coordinator Jeff Zients at a White House briefing.

"Why would people who had enough understanding of the risk to go ahead and get the primary series—why we don't have more getting the booster," Fauci, the head of the National Institute of Allergy and Infectious Diseases since 1984, said in response. "I don't have an easy explanation for that."

Fauci said that he believes that federal health data suggests that it is "stunningly obvious" that vaccinated people should get a booster dose.

"So, the only thing that we can do is to continue to come out with the data and to make sure the American public appreciates why it is so important for optimal protection to get boosted," he said.

Fauci's comments come as several recent studies have shown that natural immunity, or the immunity achieved by prior COVID-19 infection, provides effective and lasting protection. One preprint study published in Italy in late January suggested that natural immunity lasts about 18 months.

"At 18 months, 97 percent participants tested positive for anti-NCP, hinting towards the persistence of infection-induced immunity even for the vaccinated individuals," researchers wrote in a study posted to the Medrxiv website, referring to nucleocapsid, a type of protein within Covid-19.