The United States will not be vaccinating migrants detained at the border this flu season, despite that fact that at least three children have died from the flu at detention facilities since last December. 

In statements to USA Today, CBS News and NBC News, Customs and Border Patrol said "In general, due to the short term nature of CBP 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." 

According to USA Today, CBP has never administered vaccines, but those who require one can get them at nearby medical facilities. 

On August 1,  doctors at Harvard and John Hopkins wrote a letter to Congress calling for an investigation into the migrant deaths at detention facilities. The doctors focused on the deaths of migrant children who died from the flu. The letter mentioned Felipe, an 8-year-old boy who died of a Staphylococcus aureus infection as a complication of the flu in December 2018. The boy was in CBP custody when he developed symptoms.

RELATED: 6th migrant child died last year after US border detention

RELATED: DHS official defends conditions at Border Patrol stations

"Poor conditions at the facilities may be amplifying the spread of influenza and other infectious diseases, increasing health risks to children, the letter read. "Moreover, 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 "

According to the Centers for Disease Control, the flu season peaks between December and February, but cases of the virus can begin as early as October and persist as late as May. 

At least six migrant children have died while in U.S. custody or after recently leaving U.S. custody since last September.