Immunogenicity of an inactivated Japanese encephalitis vaccine (JE-VAX) in humans over 20 years at USAMRIID: Using PRNT50 as an endpoint for immunogenicity

Two hundred and ninety-three subjects received a three-dose primary JE-VAX series and had post-primary shot 3 titers within 56 days at USAMRIID from 1985 to 2005. Overall, the PRNT50 primary response rate (titer of 1:10 or greater) was 269/293 (92%). Eighteen out of 19 subjects (95%) responded with adequate PRNT50 titer within 56 days after first JE-VAX boost. Primary PRNT50 responses to JE-VAX varied significantly in response rates and in geometric means (GMT) by vaccine lot. We recommend that future vaccine studies using PRNT as an immunologic endpoint include a coefficient of variation result alongside the GMT to assist in evaluating GMT results. For subjects who responded within 56 days of primary shot 3, 50% experienced a PRNT50 decline in titer to <1:10 at 805 days and a PRNT80 decline in titer to <1:10 at 355 days. Consequently, for individuals traveling to JE endemic areas, we recommend JE-VAX boost every 2 years and for individuals working with high titers of JE virus in the lab setting, we would recommend JE-VAX boost annually.