The main purpose of booster doses in immunization schedules is to enhance and prolong immunity. Booster doses stimulate the immune system to produce a stronger and more sustained antibody response, which helps maintain protective immunity over time. This is crucial because initial vaccinations may not provide lifelong protection, and immunity can wane. Boosters ensure that memory cells are reactivated, reinforcing the immune defense against specific pathogens. Thus, boosters improve long-term vaccine effectiveness and help prevent disease resurgence.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.