Non-clinical vaccine safety assessment

As vaccines are undoubtedly classified as pharmaceuticals, they have to be submitted to strict non-clinical safety evaluation. The context of their prophylactic use requires that every effort is made to ensure their safe use. Their safety evaluation is complex as they act through a multistage mechanism in which the vaccine by itself acts as a pro-drug, antibodies and activated lymphocytes being the actual effectors. Therefore, several potential toxicities must be considered: direct toxicity of the test article, toxicity linked to the pharmacodynamic activity of the vaccine, activation of pre-existing disorders, toxicity of contaminants and impurities and other adverse reactions due to interaction between the various components. Guidelines dealing with vaccines include general guidelines applicable to all pharmaceuticals, such as ICH S6, and also more specific documents which allow some flexibility in study design. Among the various studies, if single-dose studies are generally part of the quality control test battery, repeated dose studies are pivotal. The animal model and treatment schedule selection and the parameters investigated are critical for the relevance of this safety assessment. Immunological and safety pharmacology parameters should be adapted to the specific properties of vaccines and added to this type of study. Vaccines intended for pregnant women or women of child-bearing age require embryo-fetal and post-natal studies with an adapted design to obtain appropriate fetal and maternal exposure during gestation with continuation into the post-natal period. Tests exist to detect hypersensitivity or autoimmune reactions, but require further validation. In addition to this tailor-made approach, any adjuvant or active component added to the vaccine formulation necessitate their own assessment using studies routinely performed for new drugs. From this review, vaccine toxicology would appear to be a separate discipline on its own whose predictivity will be increased by new method development.