The Incidence of Japanese Encephalitis in Taiwan—A Population-Based Study

Abstract

BackgroundA mass Japanese encephalitis (JE) vaccination program targeting children was launched in Taiwan in 1968, and the number of pediatric JE cases substantially decreased thereafter. The aim of this study was to elucidate the long-term trend of JE incidence, and to investigate the age-specific seroprevalence of JE-neutralizing antibodies.Methodology/Principal FindingsA total of 2,948 laboratory-confirmed JE cases that occurred between 1966 and 2012 were analyzed using a mandatory notification system managed by the Centers for Disease Control, Taiwan. A total of 6,594 randomly-sampled serum specimens obtained in a nationwide population-based survey in 2002 were analyzed to estimate the seroprevalence of JE-neutralizing antibodies in the general population. The average annual JE incidence rate of the group aged 30 years and older was 0.167 cases per 100,000 people between 2001 and 2012, which was higher than the 0.052 cases per 100,000 people among those aged under 30 years. These seroepidemiological findings indicate that the cohort born between 1963 and 1975, who generally received two or three doses of the vaccine and were administered the last booster dose more than 20 years ago, exhibited the lowest positive rate of JE-neutralizing antibodies (54%). The highest and second highest antibody rates were observed, respectively, in the oldest unvaccinated cohort (86%) and in the youngest cohort born between 1981 and 1986, who received four doses 10–15 years ago (74%).Conclusion/SignificanceOver the past decade, the main age group of the confirmed JE cases in Taiwan shifted from young children to adults over 30 years of age. People who were born between 1963 and 1975 exhibited the lowest seroprevalence of JE-neutralizing antibodies. Thus, the key issue for JE control in Taiwan is to reduce adult JE cases through a cost-effective analysis of various immunization strategies.

Epidemiological Features of Japanese Encephalitis in Taiwan from 2000 to 2014

“…The age of JE occurrence shifted from a younger age group to an older age group. Similar to the results of a previous study, 7 this study found that the number of JE cases reported between 2000 and 2014 in Taiwan was far below that of the pre-vaccination era, when at least 5-15 times more cases were reported annually (the average number of JE cases reported in 1968 was 200). 6 The incidence of JE was higher among people 20 years or older than it was among those under 20 years.…”

“…We did an advanced analysis and found the incidence rates of JE increased as age increased, with a peak incidence rate in the age group of 40-59 years. The previous study 7 shows that the positive rates of JE neutralizing antibodies in Taiwanese people were 74%, 63%, 55%, 54%, 68%, and 86% among 1981-1986, 1976-1980, 1970-1975, 1963-1969, and 1912-1952 cohorts, respectively. The countries neighboring Taiwan have confronted similar problems, with the majority of JE cases shifting from children to adults.…”

“…The study of Hsu and others 7 shows that the highest incidence rates of JE were in the people aged 0-29 years from 1966 to 1970; the incidence rates were similar for age groups (< 30 and ≥ 30 years) between 1992 and 2000; however, after 2001, the highest incidence rates shift to ages ≥ 30 years. The age of JE occurrence shifted from a younger age group to an older age group.…”

“…In 1998, an E/M-specific capture IgM and IgG enzyme-linked immunosorbent assay (E/M-specific IgM/IgG ELISA) for JE and dengue fever (DF) was developed by the Taiwan CDC. 7,10 Antibodies against both JE and DF are screened using hemagglutination inhibition (HI) 11,12 or an E/M-specific IgM/ IgG ELISA. 13 A differential testing algorithm was used as described in previous study, 13 in which samples that tested positive in the JE ELISA were subsequently tested using the DF ELISA, and the results of both tests were considered in the final interpretation of the data.…”

“…The vaccination was administered in two doses separated by an interval of 2 weeks, which were administered between March and May, before the epidemic season, and were followed by a booster dose 1 year later, with a final booster dose (the fourth dose) when the child entered the first year of elementary school. 6,7 Before 1987, an inactivated vaccine derived from a Nakayama strain JEV-infected mouse brain was the only vaccine used. A version derived from an inactivated freeze-dried Beijing strain was introduced in 1988, but the Nakayama strain of the vaccine has remained the dominant vaccine on the market.…”