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%.