Table D-14.1 shows the estimated incidence by age of typhoid fever cases in Africa, Asia, Latin America, and Oceania. Febrile, unrecognized cases of typhoid are assigned to morbidity category A; recognized moderate cases are assigned to category B; and recognized severe cases are assigned to category C. It is estimated that the number of mild cases of typhoid fever (probably undiagnosed) equals the number of recognized cases. It should be noted that the fatality rates in different regions vary: Table D-14.2 presents reported typhoid case-fatality rates in selected countries from 1951 to the present.
The incidence of clinically recognized typhoid fever appears to be highest in school-age children and young adults in endemic areas (Punjabi, 1984). Relatively few cases of typhoid fever are reported in children younger than 2 years of age in the same populations. There is some evidence that when these young children are exposed to S. typhi, they develop a bacteremic but clinically milder illness. Prospective studies of the age-specific incidence of the disease are needed to determine the best strategy for controlling endemic typhoid fever by vaccination. Additional information also is needed on the duration of protection afforded by such vaccines as Ty21a S. typhi and their efficacy when administered to young children.
It will be impossible to incorporate a typhoid fever vaccine into the existing World Health Organization Expanded Program on Immunization (WHO-EPI) if the primary target population is restricted to school-age children and young adults. However, if a candidate vaccine proves to be effective when given to small children and to have a long duration of protection (20 to 30 years), this situation could change. Development of a vaccine formulation other than enteric-coated capsules (which cannot be swallowed by infants and young children) will be necessary for this to happen. The calculation of vaccine benefits is based on the assumption that efforts in this direction will be successful.
Crude estimates indicate that at least 75 percent of the disease burden falls upon school-age children and young adults up to 34 years
Vaccine preventable illness is defined as that portion of the disease burden that could be prevented by immunization of the entire target population (at the anticipated age of administration) with a hypothetical vaccine that is 100 percent effective (see Chapter 7).