A ‘biological’ in WHO documents is defined as “a substance which cannot be fully characterized by physico-chemical means alone, and which therefore requires the use of some form of bioassay”.
In essence, a biological standard is a positive reference sample that is included in a biological assay to ensure that it is functioning correctly. For vaccine evaluation, this would usually be an
antibody. Biological standards are used in assays that determine the purity, potency and identity of complex biological materials such as vaccines. They also have important applications in the
standardization of nucleic acid-based tests, including real-time PCR, and serology assays used in epidemiology and clinical trials to evaluate vaccine efficacy. A biological standard that is common
to all assays allows for calibration and harmonization of assay data.