Estimating Influenza Vaccine Effectiveness Against Laboratory-Confirmed Influenza Using Linked Public Health Information Systems, California, 2023-2024 Season.

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.