The best way to prevent influenza infection is through vaccination. Evaluating vaccine performance is essentially done through two types of study: clinical trials and the test negative design, an observational study derived from the case control. While in clinical trials the sources of bias are perfectly identified and there are specific tools for assessing them, in test negative design we find very varied sources of bias and a lot of scattered information without there being a validated tool for assessing the risk of bias. The aim of this narrative review is to identify the most important sources of bias in both types of study and to contribute to the development of a risk assessment tool for test negative design studies, given their major importance in evaluating the performance of the seasonal influenza vaccine.