Abstract
Evaluating the impact of public health investments in vaccination programs is crucial for ensuring their efficiency and effectiveness. Vaccine effectiveness (VE) studies, such as those using the test-negative design, are commonly used to confirm the impact of vaccines and guide future improvements. The test-negative design, favored for its simplicity and cost efficiency, mitigates biases common in other epidemiologic study designs. However, its validity can be compromised by inconsistent symptom definitions and retrospective data application. Here we summarize recent findings that (1) highlight the need to address correlated vaccination behaviors when estimating VE, (2) suggest using negative control variables to reduce confounding, and (3) recommend accounting for infection history in VE studies to improve accuracy and reliability. These insights are important for refining VE estimation methods.