The attenuated SA14-14-2 JE virus strain, derived by serial passage in primary
hamster kidney cells, was licensed and has been distributed in areas of southern China
since 1989. Neuroattenuation of the vaccine strain was demonstrated in experimental
animals and in several large scale human studies, of which the most recent, a controlled
study of 25,000 children, found no cases of central nervous system infection in 13,000
immunized children. By some measures, the vaccine strain is overattenuated, since the
antibody responses to one dos has varied from only 85% to 100% in children receiving
vaccine containing >106.7 pfu/ml, the minimum vaccine infectious titer
specified by Chinese licensing authorities, and geometric mean titers(GMTs) in these
vaccinees generally have been low, on the order of 23∼46. Although several field
studies reported protective efficacies exceeding 95% after a single dose, based upon
these immunogenicity data, public health authorities have recommended two doses, given
in successive years, following the traditional approach of campaign vaccination each
spring. This recommendation received support in a recent case-control effectiveness
study that found an 85% vaccine effectiveness in children receiving one dose and 98%,
after two doses. The SA14-14-2 vaccine currently is distributed only in China but the
attenuated vaccine has been recognized as a potentially less costly and less reactogenic
alternative to the inactivated mouse brain-derived vaccine, which now is the only
internationally distributed JE vaccine.
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