Genetic and neuroattenuation phenotypic characteristics and their stabilities of SA14-14-2 vaccine seed virus

Japanese encephalitis (JE) live attenuated vaccine SA14-14-2 is the most widely used JE vaccine in the world. Large-scale clinical trials have demonstrated satisfactory safety and efficacy profiles. The establishment of genetic and attenuated neurovirulence characteristics and their stabilities of SA14-14-2 virus are important in relation to vaccine safety in humans. Therefore, several researchers have studied and analyzed the full-length gene sequences of the SA14-14-2 virus strain. However, sequencing results have shown a significant difference. Here, we further studied the full-length sequence of three class seed virus banks of the vaccine as well as two vaccine viruses with different passages in primary hamster kidney cells, and compared them with our original stored SA14 parent virus (low passage in mouse brain). The full-length gene sequence determined in this study indicates there were 57 nucleotide and 25 amino acid substitutions of the SA14-14-2 strain compared to its parental SA14 virus strain. The full-length sequences of the three class seed bank viruses and the vaccine virus PHKC8 were completely identical among them, but the working seed virus passaged in primary hamster kidney cells for 17 generations (PHKC17) had a single nucleotide change at the 5' NCR. Both KM and ICR mice tested by intracerebral (i.c.) or subcutaneous (s.c.) routes with the three class seed viruses and vaccine viruses with ≥5.7 lgpfu/mL remained healthy, but all the mice inoculated with the SA14 parental virus strain died as early as day 5 post-inoculation. The present study provided new information on the full-length gene sequence and attenuated neurovirulence of SA14-14-2. They can be used as a reference sequence for vaccine quality control and surveillance of neurovirulence reversion following vaccination. Moreover, the present results further demonstrated the high genetic and phenotypic stabilities of the SA14-14-2 virus, suggesting the neurovirulence reversion of the vaccine strain will be highly unlikely.