Adhering to the recommended immunization schedule for children under the age of 18 that is approved and published by the Advisory Committee on Immunization Practices (ACIP), a group under the umbrella of Centers for Disease Control and Prevention (CDC), is very important for a child's health. Appropriately timing and spacing vaccinations as per the guidelines ensures that the vaccine produces an optimal immune response. The dosage and frequency of which to administer vaccinations have been determined through clinical trials. 

Infants receive passive immunity from their mother during the final months of pregnancy, at which time the antibodies the mother has can travel across the placenta. This allows the infant to have the some antibodies as their mother, and provides protection against various diseases, including measles and rubella, for up to a year after birth. This is important because most vaccines, with the exception of Hepatitis B, are generally not administered to infants before they reach an age of six weeks. In line with this, the minimum age for Dose 1 of many vaccines, including TDaP, rotovirus, Hib, Pneumococcal, and Inactivated poliovirus is six weeks. These first dose of these vaccines is recommended to be administered at the age of 2 months, followed by a minimum interval of four weeks before the second administration at the age of 4 months. Many of the immunizations on the recommended schedule require multiple doses, usually two or more, in order for the immune system to produce an "adequate and persisting antibody response" (1). If vaccines are administered sooner than the recommended minimal interval, the immune response to the second dose will be inadequate. The first dose of other vaccines like Measles, mumps, rubella (MMR) and Varicella (VAR), are not administered until the age of 12 months. 

There are exceptions under which vaccines can be administered sooner that the recommended minimal intervals or sooner than the minimal age. If the infant develops measles before the age of 12 months, then the measles vaccine can be administered. However, this administration is not considered to be a first dose, and the first dose must be appropriately given when the child reaches the age of 12 months. Furthermore, a second exception can be made when its a matter of a few days, specifically up to 4 days, before the recommended minimums. 

Recommended Immunization Schedule for Children 0-18 Years by ACIP

(1) CDC General Rules of Immunization
(2) http://www.cdc.gov/vaccines/hcp/admin/storage/providers-role-vacc-admin-storage.html
(3) http://www.cdc.gov/vaccines/schedules/downloads/child/0-18yrs-child-combined-schedule.pdf
(4) http://www.immunize.org/askexperts/scheduling-vaccines.asp
(5) Image: http://www.cdc.gov/mmwr/preview/mmwrhtml/figures/su6201a2f1.gif