Japan's Japanese Encephalitis Risk Underestimated

According to a Case Report published by the Journal of Infection and Chemotherapy (Volume 31) in June 2025, Japanese encephalitis (JE) has emerged in a previously non-reported area in Japan, suggesting that the number of JE patients may be underestimated in Japan.

This report describes three cases of JE in a single hospital in Narita over three years.

These researchers suggest that physicians in Japan should consider JE as a differential diagnosis in 2025 when encountering cases of encephalitis or meningitis with unknown etiology during the warm season, even in areas where JE has not been previously reported.

This prefecture is located a few miles east of Tokyo, a city with over 35 million residents.

JE is an infection of the brain caused by the Japanese encephalitis virus, which is spread to people through the bite of an infected mosquito or close contact with livestock, such as birds, goats, and pigs.

In Japan, JE used to be endemic, and more than 1000 JE patients had been reported before the 1960s.

However, the introduction of the JE vaccine in 1954 and its widespread use in childhood vaccination from 1967 dramatically reduced the disease burden. As of 2022, only one case had been reported in Chiba and Ibaraki prefectures, respectively, in the past 10 years, and both cases occurred in areas famous for pig farming, far from Narita City.

As of June 5, 2025, the U.S. CDC recommends JE vaccination for specific people visiting Japan.

The CDC recommends vaccination for travelers who are moving to an area with JE, spend long periods in areas with JE present, or frequently travel to those areas in Japan.

JE vaccination is not recommended for travelers planning short-term trips to urban areas or those traveling to places with no clear JE season. 

When departing for Japan or any other JE outbreak area, such as Australia, Valneva SE's IXIARO® (JESPECT®) vaccine is commercially offered at travel clinics and pharmacies in 2025.