The number of unaccompanied migrant children testing positive for the coronavirus has jumped more than 35 percent in recent weeks as federal officials prepare for the possibility of a new immigration surge at the U.S.-Mexico border.

A total of 1,061 minors in U.S. immigration custody have tested positive since March, up from 781 cases in mid-November, according to federal records. Most minors arrived at the U.S. border already infected with covid-19, the disease caused by the virus, according to the Office of Refugee Resettlement (ORR), the federal agency that cares for minors until they are placed with a parent or guardian.

None of the minors have been hospitalized, and most have recovered and been released, agency spokesman Kenneth Wolfe said. On Monday, 118 of the 3,300 minors in federal shelters had active coronavirus cases and were isolated.

The rising number of infections poses a fresh challenge to President-elect Joe Biden as he prepares to take office in January and plans to repeal President Trump’s policies to curb border crossings. Attempted entries plunged after the coronavirus took hold across the United States, as the Trump administration invoked emergency powers to expel more than 200,000 migrants, including thousands of children.

But more recently, attempted crossings have risen as migrants fled the aftermath of powerful hurricanes in Central America and as crime, hunger and political instability continued to ravage numerous countries in Latin America. In November, a federal court barred the U.S. government from expelling minors who enter the country, which could lead to overcrowding should an influx begin anew.

“All these things are coalescing at the same time,” Carlos Holguín, general counsel for the Center for Human Rights and Constitutional Law, said after a federal court hearing Friday in Los Angeles, where authorities disclosed conditions for migrant children in federal custody. Holguin, who represents migrant children in U.S. custody, called the situation a developing “crisis.”

“The way that apprehensions have been spiking, given also the circumstances in the northern triangle in Central America, it’s something to lose sleep over,” he said. “We’re worried about it.”

U.S. District Judge Dolly M. Gee, who oversees the implementation of a 1997 federal consent decree known as the Flores Settlement Agreement — which sets basic standards for the care of migrant children — quizzed the government monitors at the hearing Friday about the rising coronavirus numbers and plans for distributing a vaccine.

Aurora Miranda-Maese, a monitor who reports to the court on conditions in the government’s shelters, said there were “several discussions going on” to prepare for a possible immigration surge.

When Gee asked whether the government had a plan to vaccinate children against the coronavirus once one becomes available, the monitor said the issue had “not come into play yet in our conversations.”

Of the 1,061 minor immigrants in U.S. custody who have tested positive since the outbreak began this year, about 600 have since recovered and been released to a parent or legal guardian, Wolfe said. He said a majority of unaccompanied minors contracted the virus before entering the United States, but court filings show that some also caught the virus inside the country.

Nearly 72 percent of those who tested positive were in Texas, where most minors tend to cross the border. But the minors testing positive were sheltered in 10 states, including Arizona, New York, Virginia and California.

At least 840 staff members have reported that they also had contracted the virus as of Nov. 12, according to federal court filings.

More than 4,600 underage migrants were taken into custody in October, up from a low of 712 in April, according to U.S. Customs and Border Protection. November figures are not yet available.

Underage border crossings remain far lower than in fiscal 2019, when a record 76,000 migrants crossed into the United States, at one point surpassing 11,000 in a single month. But the agency’s capacity to hold minors has shrunk by 40 percent, to about 8,700 beds, Wolfe said, so that the government can follow Centers for Disease Control and Prevention guidelines for social distancing.

“The fact is that covid restrictions effectively take beds offline that would otherwise be available in our network,” Wolfe said. “The safety and care of every unaccompanied alien child is our priority.”

A federal shelter contractor, speaking on the condition of anonymity because the person is not authorized to speak about internal operations, said the coronavirus outbreak has posed several challenges for shelters. Sick children must be separated from healthy children. Field trips and sports have been canceled. School is in session, but everyone is socially distanced and wearing masks.

“You can’t play soccer. You can’t do the sort of activities that are so important for these kids,” the person said, noting that ORR career and shelter staff are doing the best they can to care for children. “The people that do this for a living are taking it seriously and making the right decisions.”