5 C.F.R. § 339.205
Agencies may establish periodic medical examinations, medical surveillance, or immunization programs by written policies or directives to safeguard the health of employees whose work may expose them or others to significant health or safety risks due to occupational or environmental exposure or demands. This may include the requirement to undergo vaccination with products approved by the Food and Drug Administration (e.g., for national security reasons or in order to fulfill the duties of a position designated as national security sensitive). The need for a medical evaluation program must be clearly supported by the nature of the work. The specific positions covered must be identified and the applicants or employees notified in writing of the reasons for including the positions in the program.
5 C.F.R. §339.205