The aim of this letter is to share the discussions and proposals made by the VAC2VAC consortium on how to support the deployment of the Consistency Approach for quality control of established vaccines and thus facilitate the substitution of in vivo testing. This work answers specific questions about what does a control strategy according to the consistency testing look like and how to submit a control strategy defined according to the consistency testing. Some topics were answered in a very straightforward manner. This was the case when the deployment of the consistency approach and the corresponding changes in vaccines control strategy was supported by the generic application of procedures already described in regulatory guidelines/requirements and related to the establishment or change in the control strategy of vaccines. The application of other procedures required more specific attention and some were deeply debated before reaching a proposal. The key outcomes of this work are that robust science must be used to develop a substitution strategy and generate supportive data packages. And this good science can best occur with good scientific collaboration between the different parties involved. Therefore, early interaction between manufacturers and competent authorities before and during dossier submission is critical to success. The consistency approach, when approved and in place, will ensure vaccine products of assured  quality assurance the patient in a more efficient manner than when relying on in vivo testing. Adapting the mindset was one of the major hurdles to a progressive vision but there is now consensus between manufacturers and competent authorities to foster the elimination of in vivo testing for routine vaccine release testing.program can be found on the Cordis database from The European Commission: In that perspective, the initiative launched reflection on how to support the deployment of the Consistency Approach for quality control of established vaccines, where current quality control approaches are often relying on in vivo methods.

Discussions started with experts from industry and regulatory bodies, involved in the VAC2VAC project, on what does a control strategy according to consistency testing entail and how to submit a control strategy defined according to the consistency testing. Then the consortium organized two workshops on June 22, 2020 and June 24, 2021 regrouping VAC2VAC experts and experts not involved in the VAC2VAC consortium and who represent important stakeholders into the discussions academics, National Control Laboratories, regulators, governmental and trans-governmental organizations. The goal was to address all topics simple or complex that could be related to the change of control strategy and facilitate a common view from regulators and industries in anticipation of future submission of a method variation dossier or a marketing authorization application. Consensus was reached, and proposals were made on several topics. The aim of this document is to share these proposals to a larger audience and stimulate discussion and engagement toward the consistency approach for the substitution of in vivo testing for quality control of established vaccines.This paper summarizes the discussions and proposals made by the VAC2VAC consortium composed of experts, regulators and scientists from different institutions, manufacturers and competent authorities on how to support the deployment of the Consistency Approach for quality control of established vaccines. Summarizes the current regulatory framework taken into consideration during these discussions.