Dr. Anthony Fauci clarified Friday that health officials do not yet know if or when a COVID-19 vaccine booster shot will be necessary.

The comments from the nation’s top infectious disease expert follow estimates from the CEOs of Pfizer and Moderna that booster shots could be needed as early as September for the first wave of Americans who were vaccinated.

In an interview with “CBS This Morning,” Fauci said scientists are tracking inoculated trial participants to determine how long the vaccines remain effective, adding that the government is preparing in the meantime for the possibility that boosters will be recommended. 

“We don't know whether we will have to do it and when we will have to do it,” said Fauci, who is also President Joe Biden’s chief medical adviser. “There's estimates — ‘Well, it may be a year, it may be a little bit longer.’ The fact is we don't know. But it would really be foolish not to plan for the possibility that we might have to boost people.”

Pfizer CEO Albert Bourla told Axios this week that the drugmaker is still awaiting data, but he believes a booster shot will be needed between eight and 12 months after people have received their second vaccine dose. 

“This could become sooner than later, I believe, from September, October,” Bourla said. “But this is something again that the data need to confirm.”

Moderna CEO Stéphane Bancel made a similar prediction, telling Axios in an email that he would recommend booster shots be administered starting in September to those who received their second doses in January.

"I think as a country we should rather be two months too early, than two months too late with outbreaks in several places," Bancel said.

Fauci himself said earlier this week he believes booster shots will be necessary within a year of vaccination. 

“I think we will almost certainly require a booster sometime within a year or so after getting the primary because the durability of protection against coronaviruses is generally not lifelong,” Fauci told Axios.

But for now, Fauci said Friday people should not assume they will need a booster shot. 

“We just keep following the durability of the response,” he said. “And if the time comes when we'll need to boost people, we’ll at least have enough data because we're doing the studies now to know what we expect to see when we do boost people.”

Last week, David Kessler, the White House COVID-19 response team’s chief science officer, said boosters, if needed, would be provided for free to the American public.