Coronavirus will likely "strike again" in the fall in the United States, according to the top American scientist tackling the disease.

Dr Anthony Fauci, who is spearheading the White House Coronavirus Task Force, said he expects COVID-19 will reappear in a second wave in the October-November period.

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It would not be the first time a virus reappeared in the fall. The H1N1 outbreak in 2009 struck twice in the US, according to the Centers for Disease Control and Prevention.

After initially peaking in May and June, a second wave of H1N1 hit again in late October. In August 2010, the World Health Organization finally announced the pandemic was over.

But Fauci said if coronavirus does re-emerge, it would not be a repeat of the major crisis the country is in now.

First, the US's ability to "test, identify, isolate and contact trace will have significantly improved", Fauci said.

In addition, several clinical trials for potential COVID-19 treatments are under way, he said, adding he hopes at least one will be available by fall. There are also multiple vaccine candidates, one of which is "on track".

"So I would anticipate that, you know, a year to a year and a half, we'd be able to do it under an emergency use," Fauci said in a briefing.

"If we start seeing an efficacy signal, we may be able to even use a vaccine at the next season. So things are going to be very, very different. What we're going through now is going to be more than just lessons learned - it's going to be things that we have available to us that we did not have before," he said.

'Mitigation is actually working'

Despite the grim news that the White House is projecting between 100,000 to 240,000 deaths in the US as a result of the coronavirus pandemic even if physical distancing measures are followed, Fauci said mitigation efforts are having a positive effect.

"It's tough. People are suffering, people are dying. It's inconvenient from a societal standpoint, from an economic standpoint to go through this [distancing]. But this is going to be the answer to our problems," he said.

The US death toll from the coronavirus climbed past 4,000 on Wednesday, eclipsing China's official count.

Public health officials stressed that the number of dead could be lower if people across the country continued keeping their distance from one another.

"We really believe we can do a lot better than that," said Dr Deborah Birx, coordinator of the White House coronavirus taskforce.

That would require all Americans to take their role in preventing the spread of disease seriously, she said.

"There's no magic bullet," Birx added. "There's no magic vaccine or therapy. It's just behaviours. Each of our behaviours translates into something that changes the course of this viral pandemic."

Fauci said the estimate of as many as 240,000 people dead was "a number that we need to anticipate, but we don't necessarily have to accept it as being inevitable".