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

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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 2023-2024 influenza season (October 2023 to 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) × 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%) 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 the feasibility of assessing seasonal influenza VE using linked immunization and laboratory data from public health surveillance systems.

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