Standardization of serum neutralization assay of Japanese encephalitis virus (Nakayama NIH strain) on BHK-21 (Cl-13) cell line.

Potency testing of Japanese encephalitis (JE) vaccine has been a complex process since its inception. To overcome difficulties encountered therein, an alternative assay, serum neutralization test (SNT), using Baby Hamster Kidney 21 cell line, has been standardized. The antibody response generated against JE vaccine was quantified and the assay was found to be sensitive and specific enough with significant accuracy and precision. On analysis of cell count, a cell concentration of 1.5 x 104 was selected as the optimum, since concentrations above and below this resulted in problems of confluent monolayer formation and incomplete monolayer formation. Incubation time has also been standardized for measuring cytopathic effect (CPE). Out of the four different time points selected, 90 min was found to be adequate for 50% reduction in the amount of CPE. The accuracy of SNT assay is explained in terms of fiducial limits at 95% level. Inter- and intra-assay reproducibility testing was also performed. A comparison of potency of JE vaccine by plaque reduction neutralization test (PRNT) and SNT method was conducted and it was found that SNT can be a reliable approach for estimating the potency of JE vaccine. The results of this study throw a light on the utility of SNT assay for the potency estimation of JE vaccine in routine practice.