The U.S. refusal to completely shut down over the course of the pandemic led to an increasingly high baseline of new coronavirus infections, according to top infectious-disease expert Anthony S. Fauci, a failure he said has allowed the pathogen to mutate and spread.

Fauci, who serves as the director of the U.S. National Institute of Allergy and Infectious Diseases, said Tuesday that the nation’s “historic negative experience” with the virus was due in part to the fact that the outbreak was never fully tamed through lockdowns — ensuring that each surge in cases grew from an ever-higher starting point.

“If you recall the history, which I painfully have lived through, is that in the very beginning … what we had was a surge in the Northeast which went up and then it came down and never got to a good baseline,” he said in a conversation with Australia’s chief medical officer, Paul Kelly. The event was hosted by the Washington-based Center for Strategic and International Studies.

“The baseline was so high that that magnified the impact of the [next] surge” in the summer, he said. “And now, as we’re coming way down, we are reaching a point where we’re beginning to, if not plateau, but the slope of the deflection is starting to maybe go down a little bit more slowly, which means we might plateau again at an unacceptably high level.”

“When Australia shuts down, they shut down, and they really do get the cases, like, almost to nothing. We’ve never had that in the United States,” Fauci said.

He added that the spread of new, more contagious variants in the United States was the result of allowing the virus to run rampant for months. The United States has recorded more than 29 million cases and over 526,000 deaths.

“A fundamental tenet of virology is that viruses don’t mutate unless they replicate, and the more spread that you have in the community the greater chance you’re going to have of the initiation of and propagation of variants,” he said. “And that’s what we’re seeing in the United States.”