The spread of the Delta variant in relatively unvaccinated parts of the U.S. is getting worse.

Nationwide, the number of new Covid-19 cases is holding steady. But that steadiness hides two dueling realities, in two different Americas.

In many urban and suburban communities, Covid continues to plummet. The rate of new daily cases has fallen below three per 100,000 residents in large cities like Atlanta, Boston, Chicago, Detroit, Houston, Minneapolis, New York, Philadelphia, San Francisco and Washington. As a point of comparison, the national rate of new daily cases peaked last winter above 75 per 100,000 people.

As their populations swell nearly to prepandemic levels, U.S. immigration detention centers are reporting major surges in coronavirus infections among detainees.

Public health officials, noting that few detainees are vaccinated against the virus, warn that the increasingly crowded facilities can be fertile ground for outbreaks.

The number of migrants being held in the detention centers has nearly doubled in recent months as border apprehensions have risen, according to the Immigration and Customs Enforcement agency. More than 26,000 people were in detention last week, compared with about 14,000 in April.