Assessment of the risks associated with the use of in vivo versus in vitro potency tests for vaccines

Animal (in vivo) potency tests have been utilized for over a century in support of vaccine development and for quality testing. This is a legacy of the best science at the time of their introduction. Advances in knowledge and technology, however, have provided opportunities to utilize more sensitive assays during development and replace legacy animal tests with in vitro alternatives. This coupled with initiatives such as replacement, reduction, and refinement (the 3-R's) and quality by design (QbD) have brought industry and regulators together in the introduction of advanced vaccine control strategies. This article examines historical and current uses of animals in vaccines technical development and control, and their replacement with in vitro alternatives from a risk point of view. An overarching risk is that a vaccine tested with an alternative potency assay fails to protect its target recipient. This can be addressed from the perspective of the assay's association with the vaccine mechanism of action, and the rules used to introduce the vaccine into the patient population (e.g., specifications). Commonly understood concepts such as analytical precision play a role in risk evaluation based on its impact on the sensitivity of a test to detect meaningful product changes caused by variations in manufacture or over a vaccine's shelf life. This should be considered when evaluating solutions such as the reduction of multi-concentration (or dilution) in vivo assays to a single concentration test. While the use of animals in vaccine development will not go away all together, the paradigm must shift from in vivo tests to in vivo models. To help ensure success, principles and practices related to introduction of in vitro alternatives require global collaboration among industry, regulators, pharmacopeias, and supporting organizations.