This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
Two bodies set these schedules. They are the Immunization Practices
Advisory Committee (ACIP) of the Public Health Service, and the
American Academy of Pediatrics Committee on Infectious
Diseases. During 1994, these organizations were part of a working
group which included representatives from the American Academy of
Family Physicians which developed one schedule to incorporate ACIP and
AAP recommendations. A new schedule was been endorsed by these groups
and became effective January 1995. In modifications of the schedule
since then, sometimes one group has differed slightly from the other,
but in time they reconcile their schedules.