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immune response itself. Depression of delayed hypersensitivity (tuberculin skin test reactivity) has been observed after administration of 17D vaccine (Monath, 1984). Whether the B-cell necrosis described in YF of rhesus monkeys is associated with suppression of antibody formation is not known.

DISTRIBUTION OF DISEASE

Geographic Distribution

Yellow fever is endemic in extensive parts of tropical South America and sub-Saharan Africa. The annual incidence of officially reported YF cases varies from 50 to 300 cases in South America and from 5 to 700 cases in Africa (Table D-19.1). Investigations during several outbreaks indicate that morbidity and mortality rates are significantly under-estimated (Monath, 1984).

Bolivia, Brazil, Colombia, Ecuador, and Peru account for the majority of cases in South America. Virus reservoirs are maintained in tropical forests, such as those of the Amazon region and the Orinoco and Magdalena valleys. In some years, the virus can extend to other areas, including Central America, northern Argentina and Paraguay, and Trinidad and Tobago, causing significant outbreaks. Currently YF in the Americas occurs exclusively in its jungle form.

In Africa, YF is endemic or epidemic in 29 countries of the tropical zone between the fifteenth parallel north and the tenth parallel south. It occurs as sporadic cases of jungle YF, or as outbreaks mainly in savannah areas. Several sizable epidemics have occurred during the past 5 years, such as in Ghana (1977 to 1979 and 1983), Gambia (1978 to 1979), Senegal (1981), Ivory Coast (1982), and Burkina Faso (formerly Upper Volta) in 1983.

Disease Burden Estimates

The disease burden estimates for YF appear in Tables D-19.2. through D-19.4.

Disease burden estimates were based on reported cases of YF to the World Health Organization, multiplied by a correction factor. The correction factor was determined by dividing the number of reported cases in certain areas into more realistic assessments based on epidemiological investigation in these areas. A median value was taken from the range of values derived from one South American and five African studies (Table D-19.5).

From Table D-19.1 an average reported annual incidence was derived from the most recent 10 years for Africa and South America. These values, 181.3 and 136.3, were multiplied by the correction factor of 257.6 to get an annual incidence rate of 46,707 and 35,114 for Africa and South America, respectively.

Because of YF’s different epidemiological patterns, the distribution of cases into categories of severity and age groups was calculated



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