Introduction Taiwan introduced a two-dose inactivated Japanese encephalitis (JE) mouse brain- derived (JE-MB) vaccine into routine childhood immunization in 1968, with booster vaccination implemented in 1974 and 1983. In 2017, JE-MB vaccine was replaced by a two-dose live-attenuated chimeric vaccine (JE-CV). After implementation of JE vaccination programs, JE cases have shifted from children to adults. In this study, we described the JE epidemiology and identify high-risk groups to further inform vaccine policy.

Methodology/Principal Findings We extracted data from Taiwan’s notifiable disease surveillance database, vital statistics, and employment statistics from 2010 to 2022. Diagnosis of JE was confirmed by JE seroconversion, a four-fold increase in virus-specific antibodies, a positive JE viral nucleic-acid test, or JE virus isolation. From 2010 to 2022, a total of 313 cases of JE were diagnosed, resulting in an overall incidence rate of 0.10 cases per 100,000 person-years and a mortality rate of 0.006 per 100,000 population. Among these patients, 64% were male, and the median age was 51 years (range 0–82). Compared with people born in or after 1976 (vaccinated with four doses of JE- MB vaccine or two doses of JE-CV), those born in or before 1962 (unvaccinated) and those born during 1963–1975 (vaccinated with two or three doses of JE-MB vaccine) had a 4.2-fold (95% confidence interval [CI] 3.0–5.7) and 5.9-fold (95% CI 4.3–8.1) higher risk of JE, respectively. The relative risk of working in agriculture, forestry, fishing, or animal husbandry, compared to other occupations, was 5.0 (95% CI 3.5–7.0).

Conclusions/Significance In Taiwan, individuals born before 1976 and those employed in agriculture, forestry, fishing, or animal husbandry had a higher risk of JE. We recommend JE vaccination for people in these high-risk groups who have not been fully vaccinated or have an unknown vaccination history.

Author Summary The Japanese encephalitis (JE) virus is primarily transmitted through infected mosquito bites between animals such as pigs and birds, with humans being the dead-end hosts. JE can lead to death or long-term neurological or psychiatric complications. Vaccination is the most effective preventive measure against JE. With the implementation of comprehensive immunization programs against JE since 1960s, Taiwan has experienced a significant decline in the incidence and mortality rates of the disease. This retrospective observational cohort study utilized Taiwan’s national databases from 2010 to 2022 to investigate the epidemiology of JE and identify high-risk populations, thereby to inform vaccine policy. A total of 313 JE cases were diagnosed among Taiwanese citizens with an incidence rate of 0.10 cases per 100,000 person-years and a mortality rate of 0.006 per 100,000 population, indicating consistently low incidence and mortality rates. Several factors were statistically significantly associated with the occurrence of JE, including male gender, individuals born before 1976, residing outside the northern region, and working in occupations related to agriculture, forestry, fishing, or animal husbandry. To prevent JE, it is recommended to offer vaccination to high- risk populations who have not completed their immunization.

The authors have declared no competing interest.

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This study was approved by the Institutional Review Board of Centers for Disease Control, Ministry of Health and Welfare, Taiwan (No. TwCDCIRB112110).

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