Immunization Infrastructure Modernization Act of 2021 This bill directs the Department of Health and Human Services (HHS) to take actions to improve data sharing and other aspects of immunization information systems. These are confidential, population-based databases that maintain a record of vaccine administrations. Specifically, HHS must develop a strategy and plan to improve these systems, including systems supported by the Centers for Disease Control and Prevention; designate data and technology standards for use in these systems; and award grants to health departments and other government agencies to improve their systems. As a condition of receiving the grants, recipients must use the designated standards. However, HHS may waive this condition in some circumstances. HHS must also report to Congress on these activities.