“If we had, right from the beginning, shut everything down, it may have been a little bit different,” Dr Anthony Fauci told CNN, but added that making that decision had been complicated.
The US has recorded over 555,000 virus cases and 22,000 deaths so far.
President Donald Trump later signalled his disproval with the interview by sharing a tweet about firing Dr Fauci.
Dr Fauci has become the public face of the US fight against the outbreak, appearing alongside Mr Trump at the White House’s daily updates.
But the two have openly differed on several issues, Mr Fauci pointing out in a recent CBS interview that he takes a scientific approach while Mr Trump comes from a “hope, layperson standpoint”.
Mr Trump retweeted a post on Sunday from former Republican congressional hopeful Deanna Lorraine.
“Fauci was telling people on February 29th that there was nothing to worry about and it posed no threat to the US public at large,” it said. “Time to #FireFauci…”
But the White House later denied that Mr Trump intended to fire him. “This media chatter is ridiculous,” a spokesman said. “Dr Fauci has been and remains a trusted adviser.”
The Trump administration has issued social distancing guidance that lasts until 30 April, but there are questions over when restrictions should be lifted.
What did Mr Fauci say?
When asked about a New York Times report that Dr Fauci and other officials had suggested aggressive mitigation towards the end of February, Dr Fauci said health officials could only make recommendations from a “pure health standpoint”.
“Often, the recommendation is taken. Sometimes, it’s not. But it is what it is, we are where we are right now.”
Dr Fauci, who is leading the US response to coronavirus, added that “no one is going to deny” that logically, earlier mitigation could have saved lives.
But he said “what goes into those kinds of decisions is complicated”.
“There was a lot of pushback about shutting things down back then.”