A second wave of the novel coronavirus in the United States could be even more destructive because it will likely collide with the beginning of flu season, one of the country's top health officials have said.

Robert Redfield, director of the Centers for Disease Control and Prevention (CDC), called on Americans to use the coming months to prepare - and get their flu shots.

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"There's a possibility that the assault of the virus on our nation next winter will actually be even more difficult than the one we just went through, and when I've said this to others, they kind of put their head back, they do not understand what I mean," he was quoted as saying in an interview with the Washington Post published late on Tuesday.

"We're going to have the flu epidemic and the coronavirus epidemic at the same time," he said.

During the 2009 H1N1 swine flu pandemic, the US experienced the first wave of cases in the spring (the months of March to June), followed by a second, larger wave in the fall (September to December) and winter (December to March), during flu season.

The US has recorded more than 800,000 confirmed cases of COVID-19 since the start of the pandemic, according to a tally from Johns Hopkins University, with 44,845 deaths - the most reported in any country.

More than 2,700 Americans died within 24 hours, it said on Tuesday.

Billions of people around the world have been ordered to stay at home in recent months as governments try to prevent the highly contagious coronavirus from overwhelming healthcare systems.

The US, like other countries, has scrambled to secure enough ventilators and personal protection equipment (PPE) for medical staff while the death toll mounts.

Redfield said the virus arrived in the US just as regular flu season - which itself can strain healthcare systems - was waning.

If the two diseases had peaked at the same time, he told the Post, "it could have been really, really, really, really difficult" for health systems to cope.

Getting a flu shot ahead of the next flu season, he said, "may allow there to be a hospital bed available for your mother or grandmother that may get coronavirus".