Background: Mandatory public health reporting of influenza laboratory results and vaccine doses administered in the state of California can provide estimates of seasonal influenza vaccine effectiveness (VE).
Methods: We analyzed linked influenza immunization registry and laboratory reporting data among California residents aged ≥6 months tested for influenza during the 2023-24 influenza season (October 2023-June 2024). Individually linked laboratory reporting included influenza molecular or viral culture test result. Odds Ratios (OR) and 95% confidence intervals (CI) contrasted odds of documented 2023-2024 vaccination among persons with influenza-positive tests versus persons with negative influenza tests. VE was calculated as (1 - adjusted OR) x 100 using logistic regression, adjusting for patient age, race, ethnicity, week of specimen collection, and county of residence.
Results: Among 1,382,142 laboratory reports, 129,253 persons (9%) (129,253) had a positive influenza test result, of whom 415,390 (30%) had documented influenza vaccination ≥14 days before test date. VE against laboratory-confirmed influenza was 41% (95% CI, 40%-42%). VE was 32% (95% CI, 31%-33%) against influenza A, 68% (95% CI, 66%-69%) against influenza B, and 26% (95% CI, 24%-29%) among adults aged ≥65 years.
Conclusions: Influenza vaccination was associated with prevention of laboratory-confirmed influenza. Results demonstrated feasibility of assessing seasonal influenza vaccine effectiveness using linked immunization and laboratory data from public health surveillance systems.
Keywords: Influenza; immunization registry; surveillance; vaccine effectiveness.
Published by Oxford University Press on behalf of Infectious Diseases Society of America 2025.