Toward replacement of the monkey neurovirulence test in vaccine safety testing

For live, attenuated vaccines derived from neurotropic wild-type viruses, regulatory authorities require neurovirulence safety testing, typically using monkeys, to assure the absence of residual neurotoxicity. Ethical concerns surrounding the use of nonhuman primates in product testing, coupled with questions over its predictive value, has resulted in a concerted effort to replace monkey-based neurovirulence safety testing with more informative, validated alternative methods that include the use of lower animal species (e.g., mice and rats) and/or in vitro assays such as mutation analysis by PCR and restriction enzyme cleavage (MAPREC). MAPREC is a WHO-approved screening tool to assess reversion to neurovirulence of oral poliovirus vaccine (OPV). Monitoring the genetic consistency of OPV lots by identification and quantification of the mutational profile using the recently developed technology of massively parallel sequencing (MPS) also holds promise not only as a replacement for nonhuman primate testing of OPV lots but for other vaccines for which animal-based tests are currently performed as a measure of manufacturing consistency and freedom of adventitious virus contamination. In many cases, the greatest hurdle to availability of such alternative methods has been the process rather than the science. This report summarizes the current status of alternative methods of neurovirulence safety testing, both those validated and those currently in development.