operation was starting national immunization programs where none had existed before (Robbins, 1991). Political commitment to the goals of the EPI had to be secured from more than 90 countries. Immunization personnel had to be trained, systems had to be established to deliver and monitor immunization efforts, and adequate national and international resources had to be put in place to support the massive undertaking.

Armed with vaccines against just six diseases—diphtheria, pertussis, tetanus, tuberculosis, polio, and measles—the EPI has made remarkable strides toward achieving universal childhood immunization. By the end of 1991, an estimated 80 percent of the world's infants were reported to be vaccinated with BCG (bacillus Calmette-Guérin, the antigen used to vaccinate individuals against tuberculosis), measles vaccine, diphtheria and tetanus toxoids and pertussis vaccine (DTP), and oral polio vaccine (OPV) (Pan American Health Organization, 1993; UNICEF, 1992). Each year, EPI-sponsored immunization programs prevent some 2.9 million deaths from measles, neonatal tetanus, and pertussis as well as 440,000 cases of polio worldwide (Kim-Farley et al., 1992; Pan American Health Organization, 1993; World Health Organization, 1992).

This great achievement stands in sharp contrast to the situation in the mid-1970s, when less than 5 percent of the developing world's children were adequately immunized and when nearly 5 million children died each year from vaccine-preventable diseases (UNICEF, 1989).

Despite tremendous progress during the 1980s toward the goal of universal immunization coverage, there is concern that the success of the six-vaccine EPI effort cannot continue indefinitely (Claquin, 1989, 1990; Poore et al., 1993; REACH Project, 1990; Robbins and Freeman, 1988; Rosenthal, 1990). Each year, a new and larger cohort of children at risk for vaccine-preventable diseases must be immunized. Some 20 percent of the world's children, many in the poorest and most remote areas of the world, have yet to be reached at all by national immunization programs (Pan American Health Organization, 1993). Indeed, more than 2 million deaths and 5 million cases of disability still occur as a result of diseases that are preventable by vaccination (Pan American Health Organization, 1993; Ransome-Kuti, 1991).

It is also worth noting that the six existing vaccines offered through EPI offset only a fraction of the burden of infectious diseases affecting children in developing countries (Figure 2-1). For example, acute diarrhea causes 3 million to 5 million deaths annually and accounts for at least one third of