Identification of ongoing pregnancies using routine ultrasound procedure codes for timely surveillance of maternal vaccine uptake: A Norwegian registry study

Aim: Timely monitoring of maternal vaccination is essential for guiding immunization strategies, but ongoing pregnancies are usually not identified in health registries, resulting in delayed surveillance. We evaluated a registry-based method using routinely collected pregnancy-specific ultrasound procedure codes introduced in 2022 to identify ongoing pregnancies and demonstrate feasibility of timely vaccination surveillance, using influenza vaccination as an example. Methods: We linked population-based registry data from the Medical Birth Registry of Norway (MBRN), the Norwegian Patient Register (NPR), and the Immunisation Registry. Ongoing pregnancies in the NPR were identified using either second-trimester ultrasound procedure codes (Scenario 1) or a combination of first- and second-trimester codes with adjustments for pregnancy loss and non-attendance (Scenario 2). Estimates of vaccination uptake using ongoing pregnancies identified in the NPR as denominator were compared with estimates using birth data from the MBRN data as denominator (gold standard). Monthly and cumulative seasonal influenza vaccination uptake was assessed from October 2022 to February 2023. Results: A total of 26,996 and 27,940 pregnant women were identified using Scenarios 1 and 2, respectively, compared with 27,513 using MBRN data. Monthly vaccination uptake estimates deviated by less than 1.5% from the gold standard. Cumulative vaccination uptake was 27.1% (7328/26,996) and 26.9% (7503/27,940) for Scenarios 1 and 2, compared with 27.8% (7656/27,513) using MBRN data. Conclusions: Ongoing pregnancies can be identified through ultrasound procedure codes from administrative healthcare data, enabling robust and timely vaccination surveillance during pregnancy. The approach supports near real-time vaccination monitoring and might be applicable to broader antenatal care surveillance in general.