**5. Discussion**

Japanese encephalitis is a neuroinvasive virus, which is destructive of neural tissue. Loss of neurovirulence is an important consideration in live JE vaccine development. It is well known that the mice and Rhesus monkeys are highly sensitive animals for testing neurovir‐ ulence of JE virus. Both were used during development of live JE vaccine. The process of derivation of the SA14-14-2 strain has demonstrated fine balances between stable neuroat‐ tenuation and immunogenicity.

The principal issue surrounding the clinical application of the SA14-14-2 live virus vaccine candidate is safety, especially absence of neurovirulence and stable even passage in brain tissue. During development of the SA14-14-2 strain, it was demonstrated that unstable attenu‐ ated virus clones i.e. SA14-12-1-7 and SA14-12-1-1 could be detected by several passages in PHK cells or one i.c. mouse passage. Using this method, SA14-14-2 vaccine virus was select‐ ed for its neuroattenuation stability. To ensure vaccine safety, absence of neuroreversion after one ic passage in suckling mice is required in the quality control for vaccine production.

Compared with other live attenuated viral strains, SA14-14-2 has the following characteristics:

trols. Therefore, IL-6, IL-8, MCP-1,MCP-1α and MIP-1β may play important roles in the im‐

Besides Xu et al.[51] reported that mice immunized with SA14-14-2 virus non-structural NS1 protein, which expressed and purified from SA14-14-2 virus NS1 recombinant *E.coli* BL21 (DE3), were protected against a lethal JEV challenge (50-70% protection). Guinea-pigs vacci‐ nated with the NS1 protein presented with reduced viremia following challenge with viru‐

SA14-14-2 NS1 1 0a 0 0 0 0 0 0

Unimmunized controls 1 0 7. 5 > 579 208 0 0 0

**Table 18.** Viremia suppression in guinea pigs vaccinated with SA14-14-2 NS1 protein followed by challenge with virulent JEV. ND, Not determinedGuinea pigs were intraperitoneally (i.p.) vaccinated with NS1 protein, each 60 ug, at day 0 and 7 respectively. Fourteen days after the first vaccination, each vaccinated and unvaccinated animal was i.p.

Japanese encephalitis is a neuroinvasive virus, which is destructive of neural tissue. Loss of neurovirulence is an important consideration in live JE vaccine development. It is well known that the mice and Rhesus monkeys are highly sensitive animals for testing neurovir‐ ulence of JE virus. Both were used during development of live JE vaccine. The process of derivation of the SA14-14-2 strain has demonstrated fine balances between stable neuroat‐

The principal issue surrounding the clinical application of the SA14-14-2 live virus vaccine candidate is safety, especially absence of neurovirulence and stable even passage in brain tissue. During development of the SA14-14-2 strain, it was demonstrated that unstable attenu‐ ated virus clones i.e. SA14-12-1-7 and SA14-12-1-1 could be detected by several passages in PHK cells or one i.c. mouse passage. Using this method, SA14-14-2 vaccine virus was select‐ ed for its neuroattenuation stability. To ensure vaccine safety, absence of neuroreversion after one ic passage in suckling mice is required in the quality control for vaccine production.

challenged with virulent JEV P3 strain (each 2.5ml).a No virus detected in the undiluted serum

0 1 3 5 7 10 14

 0 0 0 0 0 0 0 0 0 2. 5 0 0 0 0 0 7. 5 20 0 0 0 0 0 17. 5 145 15 0 0 0

 0 47. 5 260 0 0 0 0 0 22. 5 318 230 2. 5 0 0 0 7. 5 225 0 0 0 0 0 0 > 750 ND ND ND ND

mune response to JE live attenuated vaccine in the humans.

**Tested groups No. animals Viremia (pfu/ml) by days postchallenge**

lent JE virus (Table 18)

198 Encephalitis

**5. Discussion**

tenuation and immunogenicity.


As for the immunogenicity of the SA14-14-2 vaccine, studies have demonstrated that protec‐ tion efficacy of the live attenuated vaccine was mediated by the presence of neutralizing an‐ tibodies and by potent cell-mediated immunity as well as the NS1 protein induced immunity, which associated with the high efficacy of the SA14-14-2 vaccine. Besides anam‐ nestic immune response will give quick rise of neutralizing antibody and provide long-term protection against JE infection among subjects who become seronegative after immunization with SA14-14-2 live vaccine [39]

Live JE vaccine (SA14-14-2) has been used in more than 300 million children since large scale production began in 1989. To date no vaccine associated JE cases has been reported in China and outside.The safety of live JE vaccine is due to a high degree of neuroattenuation and a number of stable phenotypic and genotypic characteristics. The combination of lack of vire‐ mia in the vaccinees and the absence of virus replication and dissemination in the mosquito vector make the likelihood of transmission of JE vaccine virus and the risk of environment highly unlikely. Therefore the exceptional safety, stability, immunogenicity and long-term protective efficacy present a strong case for the expended use of this live attenuated JE vac‐ cine in the world.

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