White House coronavirus advisor Dr. Anthony Fauci said Friday that the chances of scientists creating a highly effective vaccine — one that provides 98% or more guaranteed protection — for the virus are slim.

Scientists are hoping for a coronavirus vaccine that is at least 75% effective, but 50% or 60% effective would be acceptable, too, Fauci, director of the National Institute of Allergy and Infectious Diseases, said during a Q&A with the Brown University School of Public Health. “The chances of it being 98% effective is not great, which means you must never abandon the public health approach.”

“You’ve got to think of the vaccine as a tool to be able to get the pandemic to no longer be a pandemic, but to be something that’s well controlled,” he said. 

The Food and Drug Administration has said it would authorize a coronavirus vaccine so long as it is safe and at least 50% effective. Dr. Stephen Hahn, the FDA’s commissioner, said last month that the vaccine or vaccines that end up getting authorized will prove to be more than 50% effective, but it’s possible the U.S. could end up with a vaccine that, on average, reduces a person’s risk of a Covid-19 infection by just 50%.

“We really felt strongly that that had to be the floor,” Hahn said on July 30, adding that it’s “been batted around among medical groups.”

“But for the most part, I think, infectious disease experts have agreed that that’s a reasonable floor, of course hoping that the actual effectiveness will be higher.”

Public health officials and scientists expect to know whether at least one of the numerous potential Covid-19 vaccines in development worldwide is safe and effective by the end of December or early next year, though there is never a guarantee. Drug companies Pfizer and Moderna both began late-stage trials for their potential vaccines last week and both expect to enroll about 30,000 participants. 

Fauci has previously said he worries about the “durability” of a coronavirus vaccine, saying if Covid-19 acts like other coronaviruses, it may not provide long-term protection.

Health officials say there is no returning to “normal” until there is a vaccine. Fauci’s comment came a day after the World Health Organization cautioned about the development of vaccines, reiterating that there may never be a “silver bullet” for the virus, which continues to rapidly spread worldwide. The phase three trials underway do not necessarily mean that a vaccine is almost ready to be deployed to the public, the agency said.

“Phase three doesn’t mean nearly there,” Mike Ryan, executive director of the WHO’s emergencies health program, said during a virtual panel discussion with “NBC Nightly News” Anchor Lester Holt hosted by the Aspen Security Forum. ”Phase three means this is the first time this vaccine has been put into the general population into otherwise healthy individuals to see if the vaccine will protect them against natural infection.”

While there is hope scientists will find a safe and effective vaccine, there is never a guarantee, WHO Director-General Tedros Adhanom Ghebreyesus said.

“We cannot say we have vaccines. We may or may not,” he said.

On Friday, Fauci reiterated that he is “cautiously optimistic” scientists will find a safe and effective vaccine. He also reiterated that the coronavirus may never be eliminated, but world leaders can work together to bring the virus down to “low levels.”

Some of Fauci’s comments have been at odds with President Donald Trump, who has repeatedly said the virus would “disappear.”

Trump, who is seeking reelection, said Thursday that it’s possible the United States could have a safe and effective vaccine for the coronavirus before the upcoming presidential election on Nov. 3.