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lowland regions of Nepal: between 1978 and 1982, more than 2,000 cases were recognized (Joshi, 1983). In north India and Nepal, adults and children are affected equally.

Sporadic, well-documented cases of human JE have occurred in southern Thailand, Malaysia, Indonesia, and the Philippines, but attack rates in these regions are low (less than 0.1 per 100,000) and epidemic disease has never been observed (Okuno, 1978). One puzzling feature of the epidemiology of JE is that the virus can be isolated with relative ease from mosquitoes or sentinel animals in these regions, but human disease is rare (Burke et al., 1985d; Simpson et al., 1970; Trosper et al., 1980; Van Peenan et al., 1974).

Foreign visitors to epidemic regions in Asia also are at risk. Sporadic cases have occurred among tourists and within expatriate diplomatic and business communities. Epidemics of JE, involving hundreds of U.S. troops, have occurred during every recent military conflict in Asia (World War II and the Korean and Vietnam wars).

Disease Burden Estimates

Special difficulties impede efforts to determine the total disease burden of Japanese encephalitis. Incidence rates and age distribution patterns may vary dramatically within a single country. Table D-6.1 shows the incidence rates used to determine total numbers of cases and deaths due to Japanese encephalitis in endemic regions. The proportion of cases in each age group is shown in Table D-6.2. For countries for which incidence rates and age distributions were not available, estimates were based on the epidemiology of disease in surrounding countries.

The proportion of cases in each morbidity category is assumed to be the same for each age group. All cases require hospitalization. Twenty-five percent of patients die about 1 week after hospitalization, and another 25 percent develop chronic sequelae (10 percent fall into each of categories D and E, and the remaining 5 percent in category F).

Total disease burden estimates are shown in Table D-6.3.

PROBABLE VACCINE TARGET POPULATION

In most areas JE occurs in children, although as discussed above, epidemics in some regions have shown a peak incidence among adults. The target population for an improved vaccine probably would be all infants in areas of potential JE occurrence; but immediately after introduction it is likely that the vaccine also would be administered to susceptible older children and adults.

Potential target areas include Japan, China, Thailand, Korea, Vietnam, Kampuchea, Laos, Malaysia, Indonesia, the Philippines, Nepal, northern India, and Sri Lanka. To simplify the calculations, the potential target population is considered to be the entire birth cohort of all countries in affected regions (see Table D-6.4).



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