Estimating influenza vaccine effectiveness against laboratory-confirmed influenza using linked public health information systems, California, 2023–2024 season

Summary This study evaluates the effectiveness of seasonal influenza vaccination in California by analyzing data from mandatory statewide public health reporting systems for influenza laboratory results and administered vaccines. The main objective was to estimate vaccine effectiveness (VE) for the 2023–2024 season by leveraging a comprehensive linkage between the state immunization registry and laboratory reporting systems. Methods Data Sources and Setting: Data were extracted for all California residents aged ≥6 months who were tested for influenza (PCR or culture) between October 2023 and June 2024. Laboratory results included influenza type and subtype, patient demographics, and test dates. Vaccination status was determined from the state immunization registry, considering only vaccination ≥14 days prior to testing. Linkage and Analysis: Patient records were matched probabilistically. VE was calculated using a test-negative case-control design with multivariable logistic regression, adjusting for age, race, ethnicity, county, and week of testing. Vaccinated individuals had at least one documented 2023–2024 influenza dose before illness...