In this handout photo provided by the Office of Inspector General, overcrowding of families is observed by OIG at U.S. Border Patrol McAllen Station on June 10, 2019 in McAllen, Texas. (Photo by Office of Inspector General/Department of Homeland Security via Getty Images)

The U.S. won't be vaccinating migrant families in holding centers ahead of this year's flu season, despite calls from doctors to boost efforts to fight the infection that's killed at least three children at detention facilities in the past year.

"In general, due to the short-term nature of CBP holding and the complexities of operating vaccination programs, neither CBP nor its medical contractors administer vaccinations to those in our custody," a Customs and Border Protection spokeswoman said in an emailed statement.

At least three children who were held in detention centers after crossing into the U.S. from Mexico have died in recent months, in part, from the flu, according to a letter to Reps. Rosa DeLauro, D-Conn., and Lucille Roybal-Allard, D-Calif., from several doctors urging Congress to investigate health conditions at the centers.

The United States had previously gone almost a decade without any children dying while under U.S. immigration custody.

"I can tell you from personal experience that child deaths are rare events," Harvard pediatrics professor Dr. Jonathan Winickoff said in an email. Winickoff signed on to the Aug. 1 letter with forensic pathologist Judy Melinek and Johns Hopkins public health professors Dr. Joshua Sharfstein and Dr. Paul Spiegel.

They said the U.S. death rate in children from the flu is about 1 in 600,000. So far, three children have died out of 200,000 people held at detention facilities along the border, they wrote.

"When I learned that multiple children had died in detention from potentially preventable causes, it truly disturbed me," Winickoff said. "The country needs urgent answers to that question so that children stop dying in detention."

Winickoff said that current holding conditions, like being placed in close proximity to other immigrants, make it easy to spread infectious diseases from person to person. He added that contracting the flu weakens a child's immune system, making it harder to fight off other illnesses.

"A child might start out with flu but then die of another infection," he added.

If conditions don't improve, Dr. Julie Linton, chair of the American Academy of Pediatrics Council on Immigrant Child and Family Health, said more children will needlessly die.

"There's a number of things that we can do to prevent deaths and infection. Those do not include holding children in cage-like facilities and warehouses," Linton said.

Children come into holding centers with a sense of resilience, Linton said, and potentially stronger immune systems. But the stress from being held against their will can cause immune systems to tank, she said.

That, paired with unsanitary conditions, such as open toilets and "insufficient supplies" to wash hands, is a breeding ground for infection, Linton added.

The Centers for Disease Control and Prevention also recommends annual flu shots for everyone over 6 months old in the U.S.

The United States over the past year has seen an influx of people crossing the southern border, seeking relief from their home countries. From October through July, nearly 70,000 unaccompanied children were apprehended at the border, according to CBP data. In the same time period, 432,838 "family units" were taken into U.S. custody.

That's led to dangerous overcrowding of migrant facilities, where investigators for the Department of Homeland Security reported prolonged holding of children without access to showers or laundry facilities. DHS inspectors said adults were being held in standing room-only areas for up to a week and some had gone a month without a shower, contributing to unsanitary conditions and health risks, according to a July 2 report by the agency's Office of Inspector General that was obtained by NBC News.

"Flu deaths are particularly tragic in my opinion because they are almost always preventable with good public health measures," Winickoff said.

When asked about health-care access for people in custody, a CBP spokesperson said there's been a "dramatic increase" in medical personnel working along the southern border. CBP currently engages about 200 medical personnel, compared with the 20 personnel a year ago.

"Medical personnel on site are available 24/7 to provide medical diagnosis and treatment, address infectious disease issues, and coordinate referral to and follow up from local health system/emergency rooms," the spokeswoman said.

Migrants who "require vaccination" are referred to local health systems where they "may receive vaccinations ... if determined necessary," she said.

Linton acknowledged the government was employing more pediatricians, but said more legislation needs to be passed to increase care and conditions.

"When you're border patrol, whose responsibility is law enforcement, and giving them the responsibility of medical care, that's a complicated mission," Linton said. "It's critical to speak up as doctors."