New assays for the quality control of live oral poliovirus vaccine

The strains of all three types of poliovirus used in the production of live oral poliomyelitis vaccine have been shown to yield vaccines that are both immunogenic and highly attenuated when administered orally to susceptible children and adults. Experience obtained for close to four decades with the vaccines prepared from these strains indicates that laboratory and animal tests described in the World Health Organization's (WHO) Requirements for Poliomyelitis Vaccines (Oral) do ensure the consistency of virus characteristics during vaccine production. Major advances in our understanding of the molecular basis of attenuation and reversion of polioviruses resulted in the development of a new generation of tests. These include an alternative in vivo neurovirulence test in transgenic mice that express the human poliovirus receptor and a new in vitro assay (mutant analysis by polymerase chain reaction and restriction enzyme cleavage, shortly MAPREC) that assess the consistency of vaccine production at a molecular level. A WHO collaborative study was initiated in 1993 with the objective of assessing transgenic mice as potential models for evaluation of the neurovirulence of type 3 vaccines. The results of the study showed that there is a very good correlation between the TgPVR21 mouse assay and the monkey neurovirulence test. Further studies are in progress to develop a statistical model for making regulatory decisions on accepting or rejecting batches of type 3 vaccine. A WHO collaborative study was initiated in 1991 to evaluate the MAPREC assay for type 3 vaccines. The results of this study showed that the MAPREC test was a sensitive, robust and standardized molecular assay suitable for process development for new manufacturers and for monitoring the consistency of existing vaccine production. Screening single virus harvests with MAPREC before pooling them into monovalent bulk vaccine will also improve the quality of the final product.