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childhood in the poorest areas, and therefore almost entirely limited to mild or asymptomatic infection. As socioeconomic conditions improve, infection is successively delayed to later childhood, to adulthood with a high proportion of the population infected, and finally, to adulthood with smaller proportions of the population infected. As infection is delayed, an increasing proportion of cases are symptomatic; paradoxically, the clinical disease burden may increase as socioeconomic standards rise.

Actual data on disease incidence are limited; the most comprehensive source is the World Health Organization Annual Statistical Reports. This presents data on rates of reported clinical infectious hepatitis for many countries, with breakdowns by age of infection for some countries. These data do not distinguish hepatitis cases by type (A, B, or non-A, non-B), because such data are available only in the most highly developed areas. Furthermore, they provide no information about case-reporting efficiency; presumably, reporting is best in most developed areas and poorest in the least developed areas.

Disease burdens were estimated from the most recent available data for both different regions and different socioeconomic development levels within certain regions, as follows (Hadler, personal communication, 1985). Reported age-specific incidences of “infectious” hepatitis were compiled for individual countries from World Health Organization (WHO) annual statistics (1975–1981) as available. Data from serologic studies of acute hepatitis cases from the United States, western Europe, South America, and other selected areas were used to estimate the proportion of reported hepatitis cases that are due to hepatitis A. Generally, such studies have shown that a high proportion (80 percent) of hepatitis in childhood is due to HAV in all areas of the world and that a variable proportion, with a median of about 30 percent, of hepatitis cases in adults is due to hepatitis A virus. Although the latter figure varies widely and may be higher in more developed than in less developed areas, for simplicity the 30 percent figure was used throughout. These proportions were applied to available age-specific hepatitis rates for different countries. Representative rates of hepatitis A were then selected for each subregion, to be applied throughout that region. Because reporting is highly variable and underreporting is the rule, higher estimates were used where data were available from several countries in a given area. Rates for each region estimated in this manner are shown in Table D-4.1, along with the estimate of the number of cases that would result (based on 1979 population data—the last reliable data available to the author). Table D-4.2 shows the estimated numbers of cases adjusted for the 1984 population numbers. Assuming only 20 percent of cases are likely to be reported, the probable true number of cases in the developing world is estimated to be 4.765 million.

In general, developed areas in the United States, western Europe, Asia, and Oceania have modest estimated rates of hepatitis A, from 0 to 15 cases per 100,000 per year. Areas with moderate development consistently show the highest disease rates, ranging from 20 to 100 cases per 100,000 per year, and occasionally higher in parts of South and Central America, eastern Europe, the Middle East, and China. Finally,

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