Eleven major cities need to take “aggressive” steps to mitigate outbreaks of coronavirus, Dr. Deborah Birx warned.

During a phone call Wednesday with hundreds of state and city officials, Birx explained that Baltimore, Cleveland, Columbus, Indianapolis, Las Vegas, Miami, Minneapolis, Nashville, New Orleans, Pittsburgh and St. Louis were not doing enough to combat outbreaks in their cities.

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“When you first see that increase in test positivity, that is when to start the mitigation efforts,” Birx said in a recording obtained by the Center for Public Integrity. “I know it may look small and you may say, ‘That only went from 5 to 5-and-a-half [percent], and we’re gonna wait and see what happens.’ If you wait another three or four or even five days, you’ll start to see a dramatic increase in cases.”

While Birx noted encouraging declines in positive test results in the Sunbelt states, she warned that the virus would start moving north.

“What started out very much as a southern and western epidemic is starting to move up the East Coast into Tennessee, Arkansas, up into Missouri, up across Colorado, and obviously we’re talking about increases now in Baltimore,” Birx said. “So this is really critical that everybody is following this and making sure they’re being aggressive about mitigation efforts.”

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The CPI reported that some of the cities Birx cited as most at-risk were not actually involved in the call, though. A spokesperson for Cleveland said they were not on the call, and a Baltimore health department leader said they didn’t know about it.

The warning arrives a week after an article published by the CPI revealed the White House compiled a report noting 18 states were in a coronavirus “red zone.” Those states exceeded 100 positive cases out of 100,000. The CPI claimed the White House did not publicly disclose the information.

As of Friday afternoon, there were more than 4 million confirmed cases of COVID-19 in the United States, and the virus was responsible for at least 144,552 deaths nationwide.

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“This is a pandemic. You cannot hide it under the carpet,” said Bill Hange, a Harvard epidemiologist. “The best way to deal with a crisis or a natural disaster is to be straight with people, to earn their trust and to give the information they need to make decisions for themselves and their communities.”