U.S. Customs and Border Protection agents and their medical contractors do not administer influenza vaccines to migrants being held at detention centers along the U.S.-Mexico border, reports said Tuesday.

“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,” the agency told The Hill in a statement. Children receive the flu vaccine once they are transferred from CBP to Health and Human Services (HHS) custody, the CBP spokesperson said.

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Earlier this month, doctors from Harvard, Johns Hopkins and other medical groups wrote a letter to two top House Democrats, asking for a congressional investigation into conditions at border detention centers ahead of flu seasons. The doctors argued a “rapid turnover of detainees increases the spread of influenza” at centers.

The doctors said at least three of the six children who died in CBP custody this year were at least partially affected by the flu, according to their autopsy reports.

“These tragic deaths appear to represent more than half of child deaths in the last year in these immigration facilities and to reflect a rate of influenza death substantially higher than that in the general population,” doctors’ letter said. “Another influenza season is around the corner. … Timely action is critical.”

Children who fall ill with influenza at detention centers appear to be at a higher risk of complications due to an alleged lack of protocol in regard to isolating and separating patients to minimize the spread of infectious disease, the doctors said. They recommended CBP administer flu shots at the centers to children as quickly as possible to help protect them against the illness.

Reps. Rosa DeLauro, D-Conn., and Rep Lucile Roybal-Allard, D-Calif., in turn wrote to the U.S. Department of Health and Human Services (HHS) and the Department of Homeland Security (DHS), which oversees Border Patrol, echoing the concerns from doctors that poor conditions in border facilities that may be exacerbating the spread of infectious disease.

Their offices demanded a response by Aug. 30, given that the start of flu season is approaching.

CBP has increased the number of medical staff contracted to work in facilities along the U.S.-Mexico border, the agency spokesperson told The Hill. Migrants receive an initial medical assessment upon entering CBP custody. If it is determined necessary, migrants may be referred to the local health system and receive vaccinations at a local medical facility, the spokesperson said.

Approximately 200 medical staff are currently contracted to work along the southwestern border. Most detention centers in high volume areas, including the Rio Grande sector, have “24/7 medical support available on site,” the agency spokesperson added.

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The Centers for Disease Control and Prevention (CDC) recommends that everyone over 6 months old receive annual flu vaccinations. The agency recommends the vaccine be administered by the end of October in order to prepare for a flu season that peaks between December and February.