The geographic distribution of dengue infection and disease has increased steadily since World War II, and the virus may be more widely circulating now than at any time in history. Certainly more people are infected annually than at any previous time (Halstead, 1980b). The virus is endemic or enzootic in the warm areas of virtually all tropical countries. The vector, Aedes aegypti, does not thrive at altitudes above 1,000 meters; hence, dengue is a lowland or coastal disease. The virus has the potential to spread to temperate zones during summer months, as in the recent dengue epidemics in Queensland, Australia. Much of the southern United States is receptive to dengue transmission.

DHF/DSS is endemic in all tropical Southeast Asian countries. A single large outbreak (116,000 hospitalizations) occurred in Cuba in 1981.

An estimated 1.5 billion persons live in countries with dengue activity (Halstead, 1980b). In dengue endemic areas of tropical Asia, virtually all adults have flavivirus hemagglutination inhibition (HI) antibody, presumably dengue in origin. Therefore, a conservative estimate is that in endemic areas, dengue viruses infect 10 percent of the susceptible population per year. Infections at this rate effectively mean that most adults are immune. If it is estimated that 40 percent of tropical populations are children 15 years and younger (600 million children), then there are about 60 million dengue infections per year. These figures ignore the circulation of multiple types, which will cause an upward revision of the estimate.

As shown in Table D-1.1, cases are assumed to occur only in the three younger age groups, with 90 percent in the two youngest groups. All cases fall into acute morbidity categories A, B, and C. Illness usually results in a visit to the physician, drug prescription for symptomatic relief, and treatment at home. Hence, cases of dengue fever are assigned to categories A and B. For all cases in category A, there is likely to be an equal number of asymptomatic infections (22.5 million). In tropical Southeast Asia, DHF/DSS has produced 1.3 million hospitalizations and 23,000 deaths in a 30-year period. DHF/DSS incidence is increasing, with a mean annual incidence for the past 5 years of about 60,000 hospitalizations and 1,500 deaths (Halstead, personal communication, 1985).

The potential target population for a safe and effective dengue vaccine includes (1) infants and children in DHF/DSS endemic areas and (2) infants and children (and initially adults) in countries with