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. More than 7,500 new coronavirus cases have been reported in the centers over that same period, accounting for more than 40 percent of all cases reported in ICE facilities since the pandemic began, according to a New York Times analysis of ICE data.