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Fox News medical contributor Dr. Marc Siegel told "Bill Hemmer Reports" Thursday that U.S. officials should keep an eye on the spread of coronavirus in South America as they prepare for a possible second wave of the pandemic in the fall and winter.

"Today, I don't like what's coming out of Brazil," Siegel told host Bill Hemmer. "They have close to 300,000 cases in Brazil ... but they've had the highest number of cases on record over the last 24 hours, close to 20,000 cases; and Argentina is gearing up."

Siegel noted that South American countries are currently experiencing conditions that would be the equivalent of late November in the Northern Hemisphere.

The NYU Langone internist added that Australia is having better luck containing the spread of COVID-19, but explained that the government there instituted a lockdown very early on.

"So if I see South America, which is not on lockdown, seeing more cases [in their] late fall, that is when our second wave would be," Siegel said. "We are looking to them to see what we could anticipate here and their fall is deep fall now and they are seeing a surge of cases."

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Siegel told "Tucker Carlson Tonight" Wednesday that he believes lockdown orders don't work unless they come into effect before a virus is widespread in a particular location. He clarified Thursday that while a second wave of coronavirus in America is not certain, public health should remain vigilant.

"[It] is an evolving situation," he said. "We have to be looking at it day by day to figure this out."