For many vaccines, clinical efficacy studies are (or are gradually) becoming unethical or unfeasible to conduct and immunogenicity endpoints are used as surrogates for demonstrating vaccine efficacy. Thus, bioassays which measure immune responses have played an increasingly important role in the clinical development of vaccines. Yet, assays used in clinical studies are often validated without careful consideration of their use in each individual clinical study. As a result, a “validated” assay may be found not fit for use after the clinical study is completed, severely impacting the interpretability of the clinical study results. In this presentation, several statistical issues and challenges related to the development and validation of clinical assays that have been identified over the recent years will be discussed using real examples.