The critical need for system interoperability and robust information infrastructure in public health was highlighted during the COVID-19 pandemic. An assessment of the evolving interoperability between immunization information system (IIS) in a state-based public health agency and electronic health records (EHRs) including pandemic-driven evolution/use was conducted. The Minnesota Immunization Information Connection (MIIC), the IIS for Minnesota (US) supports interoperability with EHRs using HL7v2.5.1 standards-based queries. Structured interviews were conducted with 28 experts across 12 healthcare systems and public health clinics (n=286 sites) between April - July 2022. Though all reported use of MIIC, most (83%) had MIIC integration within their EHRs, and high EHR queries to MIIC (∼6 million/month), numerous organizational/technical barriers were identified including standard vaccine-naming need in EHRs, app access issues, limited resources and informatics-staff shortage in public health. Results underscore vital role of IIS, on-going interoperability evaluation to address issues and promote standards-based bi-directional EHR-IIS data exchanges.