Flu season is coming. But for migrants in U.S. custody, vaccines won't be provided, despite the fact that influenza has contributed to the deaths of at least three detained migrant children since December.

"In general, due to the short term nature of Customs and Border Protection holding, the time the vaccine takes to begin working, and the complexities of operating vaccination programs, neither CBP nor its medical contractors administer vaccinations to those in our custody," CBP said in a statement to USA TODAY.

CBP has never administered vaccines, but people who require vaccines may receive them at a local medical facility, if necessary, CBP said. In circumstances with public health considerations regarding vaccination, the agency coordinates with local health authorities and the CDC for support, CBP said.

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Medical professionals, however, say CBP needs to do more to prevent the flu.

"Death due to influenza is a rare event among children in the United States, and it should be an equally rare event among children in detention facilities run by the United States government," Dr. Paul Spiegel, director of the Center for Humanitarian Health at Johns Hopkins, told USA TODAY.

This month, doctors associated with Johns Hopkins and Harvard universities, including Spiegel, wrote a letter to Congress expressing concern about the threat of influenza inside detention centers.

“We suspect that the Department of Homeland Security and Department of Health and Human Services may not be following best practices with respect to screening, treatment, isolation and prevention of influenza," the authors wrote.

The letter cited autopsy reports of detained migrant children ages 2, 8 and 16 who died, in part, because of the flu. The authors noted that these influenza deaths occurred at a rate substantially higher than that in the general population.

"A decision not to provide vaccines to people at risk is a decision to put them further in harm’s way," said Dr. Joshua M. Sharfstein, vice dean for Public Health Practice and Community Engagement at Johns Hopkins, who also signed the letter.

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Dr. Judy Melinek, a forensic pathologist who reviewed the autopsy reports, urged lawmakers to ensure that CBP follows best medical practices to prevent outbreaks.

"This is especially alarming considering the crowded conditions, which can increase transmission of respiratory illness, and the policy of family separations, which deprives children of an immediate caregiver. If there is an epidemic as a result of these policies, it will be on their hands," Melinek said.

It's not just influenza that's a problem. A government watchdog report this month found that facilities housing undocumented children in Arizona, California and Texas "did not always comply with health and safety requirements." Some children in government custody have even reported sexual abuse and harassment, according to a joint investigation by PBS and The Associated Press.

At least six migrant children who were in U.S. custody or recently left have died since September 2018.

Follow Grace Hauck on Twitter at @grace_hauck.