4. Quality standards in the consistency approach
Testing is only one component of an operational quality system, which includes careful validation and maintenance of the manufacturing process that produced lots of vaccine shown to be clinically safe and effective. Nowadays it is a well-established philosophy that the quality of a vaccine is not ensured by testing alone, but rather built through the strict application of the quality assurance system. The consistency approach reinforces this vision and extends it to other activities in the vaccine life-cycle, such as the field history of the vaccine, established before licensing by clinical studies of more than one lot of product on trial, and after licensing, by programs of pharmacovigilance. The importance of complete and accurate written procedures and the existence of a system that evaluates the effect of changes in the production process cannot be over emphasized.
As mentioned above, although release specifications are useful tools to establish consistency of manufacturing, other in-process characteristics and parameters measured on the intermediates and the final product may also contribute to the assessment of consistency. Manufacturers should have established, as part of a quality assurance system, procedures and set control limits (e.g. alert limits, action limits) for controlling the performance of any meaningful parameter over time. Release specifications should not be confused with quality control limits. Release specifications are defined during product development and tend to be less conservative than some quality control limits, although both are based on the accumulated experience of both the manufacturers and the regulators. Sometimes both types of parameters are set on a provisional basis at the time of licensing and can only be fully defined after a sufficient number (decided on a case-by-case basis) of lots are manufactured. For the same parameter, the buffer zone between quality control limits and release specifications allow the manufacturer to investigate deviation from control limits and correct their root causes without sacrificing numerous batches. Additionally, the existence of quality control limits, in addition to specifications, allows the manufacturer to improve consistency over time, without the need for a formal change in specifications that could entail the performance of additional clinical or animal studies.
Extended acceptance of the consistency approach will require a profound review of the current system of international standardization [11]. A fundamental first step in international harmonization is the recognition of the validity of the proposed approach. Once the approach has been endorsed internationally, publication of guidelines explaining its operative details is of utmost importance.
One of the most misunderstood parts of the consistency approach is the role of physical standards in potency testing. Because the consistency approach intends to establish a link between the clinical lots and subsequent batches of vaccine, it is often assumed that there is a need for a stable clinical lot that could be used as standard or control. In reality, what is assessed in the consistency approach is the similarity of the potency or activity of every batch manufactured after licensing to that of one or more lots shown to be safe and effective. For this purpose, if the manufacturer and the regulatory authority so decide, limits of acceptance can be calculated relative to a standard vaccine of known behavior. This batch of vaccine may or may not be a clinical lot, and can be replaced whenever it becomes necessary, provided that the activity of the new reference is adjusted with respect to that of the old reference. This