Natural infection with Japanese encephalitis virus among inhabitants of Japan: a nationwide survey of antibodies against nonstructural 1 protein.

In Japan, only a few cases of Japanese encephalitis presently occur annually, raising arguments about the necessity of vaccination. Here, using a novel immunoassay, we obtained natural infection rates for 2001 of 0.2-3.4% in eight selected prefectures across Japan. Our results provide direct evidence of the risk of reemergence.