Development of neutralization tests using single-round infectious particles and cytopathic effect as an alternative method for measuring antibody titers against Japanese encephalitis virus in national epidemiological surveillance program of vaccine-preventable diseases in Japan.

The "National Epidemiological Surveillance of Vaccine-Preventable Diseases" is conducted annually in Japan for various infectious diseases. A susceptibility survey for Japanese encephalitis was conducted by measuring antibody titers using a focus-forming assay. However, this method is complicated to operate and hard to count small focuses; therefore, the development of new measurement methods is required. In this study, we developed and evaluated a single-round infectious particles (SRIPs) assay method and a cytopathic effect (CPE) assay method. The SRIPs assay showed a strong correlation (R = 0.94) with the focus-forming assay. The SRIPs assay method is not only simple and can be expected to improve accuracy, but also has the advantages of extremely low pathogenicity and safety for the operator. The CPE assay also showed a strong correlation (R = 0.80) with the focus-forming assay. It has the advantage of requiring almost no reagents and is easy to interpret. Both SRIPs and CPE assay methods can be used as alternatives to the focus-forming assay method, but further characterization of each method is necessary before one is selected for routine use.