LONDON, ENGLAND - JULY 30:  A woman wearing a surgical mask to prevent the transmission of airborne infection walks in Westminster on July 30, 2009 in London, England. Figures released by the Health Protection Agency show that by July 19, an estimated 100,000 Britons have been infected with swine flu and there have been 26 deaths in England linked to the disease. Over 700 people worldwide have died after contracting swine flu which is spreading faster than any other flu pandemic.  (Photo by Oli Scarff/Getty Images)
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(CNN) —  

A group of physicians has volunteered to vaccinate migrants against the flu for free, but US Customs and Border Protection is all but certain to say no to the offer.

“We haven’t responded, but it’s not likely to happen,” a CBP official told CNN.

“As a law enforcement agency, and due to the short term nature of CBP holding and other logistical challenges, operating a vaccine program is not feasible,” according to a statement from the federal agency.

The offer of a vaccination clinic came in a letter on November 5 to federal officials from seven doctors affiliated with Doctors for Camp Closure.

“We’re disappointed and frustrated,” said Dr. Luz Arroyo, who signed the letter and practices family medicine and psychiatry in Sacramento. “But we’re not going to give up that easily and will continue to request access.”

The group, which formed about three months ago and has about 2,000 physician members, outlined a plan to first vaccinate 100 migrants, and then expand to vaccinate the majority of migrants.

“We implore you to allow our volunteer physicians to hold our requested influenza vaccination clinic,” the doctors wrote.

NBC News first reported the letter.

In the 2018-2019 flu season, at least three children died of the flu while in CBP custody.

An infectious disease expert and longtime adviser to the US Centers for Disease Control and Prevention noted that this year, the flu has started early and is well underway, and that one of the major circulating strains is Influenza B, which tends to hit children especially hard.

“We have an early and very active influenza season already in the United States, and given the history of influenza and the illness and deaths caused by influenza last year, I’d think CBP would do everything it can to provide protection against influenza as quickly as possible,” said Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases.

This isn’t the first time doctors have asked CBP to vaccinate migrants against the flu. In August, a group of physicians from Harvard and Johns Hopkins wrote a letter to members of Congress stating that vaccination should be offered to all detainees “promptly upon arrival.” The legislators forwarded the letter to federal officials.

“I would have thought that CBP would have addressed this over the summer and we would now have a system in place,” said Schaffner, a professor of preventive medicine at Vanderbilt University Medical Center. “Vaccination is not complicated. We conduct mass campaigns all the time.”

The letters from both groups of physicians were sent to Kevin McAleenan, then acting director of the Department of Homeland Security, and Alex Azar, secretary of the Department of Health and Human Services. McAleenan has since left his post.

In the first seven months of 2019, more than 600,000 migrants were apprehended crossing the US southwest border. CBP policy, rooted in legal agreements, aims to move both children and adults out of its custody within 72 hours.

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Government reports show, however, that both children and adults have been held for considerably longer than 72 hours in crowded conditions.

“The facilities in which they’re detained could not be better designed for the spread of influenza,” Schaffner said.

When migrants leave CBP custody, they often go into the custody of the Department of Health and Human Services and ICE, both of which “have comprehensive medical support services and can provide vaccinations as appropriate to those in their custody,” according to the CBP statement.

CNN’s Kristen Rogers contributed to this report.