Maternal Immunization Enhancement Act This bill establishes several administrative requirements relating to vaccination rates among pregnant women enrolled in Medicaid and the Children's Health Insurance Program (CHIP). Specifically, the Centers for Medicare & Medicaid Services must (1) issue guidance to states with best practices for increasing vaccination rates for certain vaccines that are recommended by the Centers for Disease Control and Prevention, and (2) include a quality measure regarding prenatal immunization status for pregnant women under Medicaid and CHIP. Additionally, the Government Accountability Office must report on vaccination rates among pregnant women enrolled in Medicaid and CHIP, including barriers and demographic data.