(CNN) US Customs and Border Protection will not vaccinate migrants, even though three children who had been in US custody died after contracting the flu.

The cases all occurred since December.

"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," according to a statement Tuesday from CBP.

Migrants are supposed to held in CBP custody for 72 hours or less , but often remain there for longer.

After leaving CBP custody, children without parents are sent into the care of the US Department of Health and Human Services, where flu vaccines are distributed, according to Evelyn Stauffer, a spokeswoman for the Office of Refugee Resettlement, a part of HHS.

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