Birx warns 9 cities, California's central valley about increasing coronavirus cases - CNN

She said Los Angeles may have seen improvements but that there's significant movement of the virus up California's Central Valley.

Despite Birx's quick reference to 10% positive test rates, it's not entirely clear which states the task force has designated as "red," "yellow" or "green," how often that label may change or what the criteria may be for the designation because the panel hasn't released the information on the classification system.

In another private call last month, Birx warned of a concerning rise in coronavirus cases in 12 cities, including Miami, New Orleans, Las Vegas, San Jose, St.

Louis, Indianapolis, Minneapolis, Cleveland, Nashville, Pittsburgh, Columbus and Baltimore.

Birx said Wednesday that she has crisscrossed the country over the past five or six weeks, driving about 6,000 miles, and has seen a few trends for herself.

"We've really seen that America is on the move and people are going on vacation," she said.

She urged officials on the call to draw attention in their state to the dangers of exposing older family members to returning vacationers because of wide asymptomatic spread.

Birx again blamed "super spreader" events for driving the virus and said mitigation efforts should continue.

Birx ended the call on a hopeful note.

"We are watching each county, each city across the United States very carefully and are providing the best advice we have based on models ...