42 C.F.R. § 61.5
In order to qualify for a regular fellowship, an applicant must:
(d) If a citizen or noncitizen national of the United States, sign and file with the Surgeon General the following statement:
I do solemnly swear (or affirm) that I bear true faith and allegiance to the United States of America and will support and defend the Constitution and laws of the United States against all its enemies, foreign and domestic.