The U.S. won’t be vaccinating migrant families in holding centers ahead of this year’s flu season, despite calls from doctors to boost efforts to fight the infection that’s killed at least three children at detention facilities in the past year.

“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,” a Customs and Border Protection spokeswoman said in an emailed statement.

At least three children who were held in detention centers after crossing into the U.S. from Mexico have died in recent months, in part, from the flu, according to a letter to Reps. Rosa DeLauro, D-Conn., and Lucille Roybal-Allard, D-Calif., from several doctors urging Congress to investigate health conditions at the centers.