deaths. The derivation of both sets of estimates are discussed in Appendix C.
In developing countries, the incidence of disease associated with enterotoxigenic E. coli appears to be highest in children during the first 2 years of life, when one or even two episodes per year per child have been noted. The incidence remains high for the first 5 years of life and moderately high for children 5 to 9 years old (Black, personal communication, 1984). Although older children and adults also suffer from E. coli diarrhea, partial immunity does appear to develop after childhood. Thus, the probable vaccine target population would be children within the first 6 months of life. The vaccine could be incorporated into the World Health Organization Expanded Program on Immunization (WHO-EPI) with delivery at the earliest current age of vaccine administration.
Travelers to developing countries, primarily adults, constitute a second potential vaccine target group.
Two considerations influence the estimation of vaccine preventable illness for vaccines against enterotoxigenic E. coli. First, knowledge is incomplete regarding which components should be included in a vaccine to cover all, or a high proportion of, possible natural challenges. Known adhesion determinants (which are relatively few in number) are encountered in about 65 to 75 percent of LT+/ST+ strains (which cause most of the severe disease). The percentage is lower (up to 25 percent) in strains that produce only LT or ST, but these strains probably cause most of the cases (Levine et al., 1983). Thus, it is difficult to estimate the coverage of strains that a vaccine could currently achieve. Work is currently under way to identify the adhesion determinants or other colonization factors present on strains lacking CFA I and CFA II or other known determinants. Thus, in a few years it may be possible to better identify the determinants that should be included in the vaccine. For the calculations, it is assumed that by the time of licensure a vaccine could probably be formulated with a reasonable number of purified components (CFA I and CFA II, adhesion factors, and possibly other components, e.g., toxoids) that would cover about 70 percent of strains. (It is recognized that this assumption is more optimistic than some researchers believe.)
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).