The Biden administration is preparing to begin offering coronavirus vaccine to migrants in U.S. custody along the Mexico border, where illegal crossings are at their highest levels in over two decades and health officials are struggling with soaring numbers of infections, according to two Department of Homeland Security officials with knowledge of the plan.

Until now, only a limited number of migrants have received vaccine while held in longer-term U.S. Immigration and Customs Enforcement detention facilities. Under the broad outlines of the new plan, DHS would vaccinate migrants soon after they cross into the United States as they await processing by U.S. Customs and Border Protection.

Vaccine would be provided to those facing deportation as well as migrants likely to be released into the United States pending a court hearing, said one of the two officials, who spoke on the condition of anonymity because they were not authorized to discuss the tentative plan. Migrants who are quickly sent back to Mexico under the Title 42 public health law would not be offered a dose, at least during the initial phase, the person said.

The Biden administration plans to use the Johnson & Johnson vaccine, whose single-dose regimen is better suited to a transient population that may not be able to coordinate a second shot, the DHS officials said.

In a statement, DHS spokesperson Meira Bernstein said the department will “continue to monitor and reassess” its pandemic protocols, but she insisted that no decisions have been made. “At this time, our protocols have not changed,” she said.

But officials familiar with the plan say the border vaccination campaign could help mitigate the delta variant’s aggressive spread on both sides of the U.S.-Mexico border. In Texas, where the daily case count exceeded 10,000 last week for the first time since February, Gov. Greg Abbott (R) and other Biden opponents have blamed the president’s border policies for spreading the virus. On Tuesday, a federal court blocked Abbott’s attempt to restrict bus companies and other transportation firms from carrying migrants.

In July, 210,000 migrants crossed into the United States along the southern border, the highest one-month total in 21 years, according to preliminary DHS estimates. David Shahoulian, the assistant secretary for border and immigration policy at DHS, outlined the administration’s public health concerns in a court filing Monday.

“The rates at which encountered noncitizens are testing positive for COVID-19 have increased significantly in recent weeks,” Shahoulian said. “And although the rate of infection among CBP officers had been declining, this rate recently began increasing again, even though the percentage of officers and agents who have been fully vaccinated has grown significantly since January.”

“This has led to increasing numbers of CBP personnel being isolated and hospitalized,” said Shahoulian, whose declaration warned of potentially dire consequences if the Biden administration is no longer able use the Title 42 public health code to rapidly return migrants to Mexico.

Immigrant advocates are suing the Biden administration to halt the turnbacks, arguing that ­Title 42 denies asylum seekers the right to apply for protection under U.S. law.

CBP was holding more than 10,000 migrants in custody as of Aug. 1, Shahoulian told the court, nearly eight times its covid-adjusted capacity. The agency has released more than 100,000 recent border crossers in the Rio Grande Valley of South Texas since October, including 9,000 last week.

Those familiar with the new vaccination plan said it makes sense for the government to provide shots to people who are already in U.S. custody, reducing the risk that they could spread the virus in their home countries or in the United States.

With the U.S. government holding far more vaccine doses than it can distribute domestically, the Biden administration has ramped up global vaccine donations, and officials said Tuesday that they have sent more than 110 million doses to at least 60 nations.

Paul A. Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said the plan to expand vaccinations along the border is “a great idea.”

“If they have people under their care and consider part of caring for them making sure they’re immune to the virus, I think that’s responsible,” Offit said. “The advantage of the Johnson & Johnson vaccine is that its single dose is for populations who are transient and less likely to get that second dose, so it makes abundant sense.”

ICE officials are preparing to send at least 500 officers and personnel to temporarily assist CBP in border sectors where overcrowding is most acute. ICE medical staff would coordinate the vaccine effort, according to two officials with knowledge of the tentative plan.

About 20,000 detainees in ICE custody have received vaccine doses so far, according to the latest government figures. Advocates for immigrants and ICE have battled in federal courts for months over the care of detainees during the pandemic. More than 1,000 immigrants were actively infected as of Aug. 1, among more than 22,500 confirmed cases since the pandemic began. Nine have died of covid; the most recent death was a 57-year-old Mexican national in February.

The Trump administration created a plan to provide vaccinations directly to the Bureau of Prisons, which had vaccinated more than half of its population as of May, according to the Government Accountability Office, and currently has more than 300 inmates with active cases.

An official told the GAO in July that the Bureau of Prisons had offered vaccine to more than 95 percent of all inmates, and followed up with those who declined it.

But Trump officials did not make those same provisions for immigrant detainees, instead leaving it up to local and state governments to offer them vaccines.

Federal judges in New York and California have expressed frustration in recent months over the slowness of ICE’s vaccination process.

Retired magistrate judge Patrick Walsh, who was appointed as a special master in a federal lawsuit over the care of immigrant detainees, said in a report that immigration detention facilities were “in the midst of an unprecedented surge in cases,” including 2,350 new infections in the first three weeks of May.

“Vaccinating subclass members would significantly reduce the risk of them becoming sick and/or dying,” Walsh told the court in his May 21 report. “The fact that the federal government has not adopted a plan to insure that they are vaccinated runs counter to the science.”

At his urging, Judge Jesus Bernal required the government to make vaccine available over the next month to immigrant detainees at risk of dying or falling seriously ill from covid.

But in a July 31 report, Walsh said that ICE was still struggling to vaccinate migrants — and that the agency was holding too many migrants with underlying health conditions. He said about 46 percent of ICE detainees who were offered vaccine had refused it during the past month.

Walsh urged Bernal to order ICE to deliver a letter to each vulnerable immigrant detainee, in their language, explaining the benefits of the vaccine and urging them to take it.