What is meant by “minimum intervals” between vaccine doses?

Vaccination schedules are generally determined by clinical trials, usually prior to licensure of the vaccine. The spacing of doses in the clinical trial usually becomes the recommended schedule. A “minimum interval” is shorter than the recommended interval between doses, and is the shortest time between two doses of a vaccine series in which an adequate response to the second dose can be expected. The concern is that a dose given too soon after the previous dose may reduce the response to that dose. The minimum spacing between doses is generally included in the ACIP recommendation for that vaccine which can be found at www.cdc.gov/vaccines/hcp/acip-recs/index.html. In addition, an extensive listing of recommended and minimum intervals and ages for vaccination can be found in the ACIP “General Best Practices Guidelines for Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html#, Table 3-2.