The Childhood Immunization Schedule: Why Is It Like That?
Q1: How is the childhood immunization schedule determined?
A: The schedule is determined by top experts in the fields of epidemiology, infectious disease prevention, and immunology to best protect U.S. children against vaccine-preventable diseases. The schedule is evaluated each year based on the most recent scientific data available, and adjustments are made as appropriate. It is approved by the AmericanAcademy of Pediatrics, the Centers for Disease Control and Prevention and its Advisory
Committee on Immunization Practices, and the American Academy of Family Physicians.
Updated recommendations are announced each January.
Q2: How are the timing and spacing of the shots determined?
A: Each vaccine dose is scheduled for the age range that is considered optimal for producing the best immune system response, balanced with the need to provide protection to infants and children at the earliest possible age. Doses of some vaccines must be spaced a certain amount of time apart to create a protective response.
http://www.vaccinateyourbaby.org/pdfs/Vaccine_schedule.pdf