- Dr Deborah Birx issued the grave warning Wednesday during a call with hundreds of state and city leaders
- The 11 cities include: Baltimore, Cleveland, Columbus, Indianapolis, Las Vegas, Miami, Minneapolis, Nashville, New Orleans, Pittsburgh and St Louis
- Dr Birx warned that the outbreak currently plaguing cities in Sunbelt states, like Phoenix and San Antonio, was now moving north
- She warned leaders of the 11 cities that they should take ‘aggressive’ steps to stem their current outbreaks
- Her warning came as the number of cases reported in the United States passed 4 million on Thursday