Oral vaccines are particularly attractive for immunizing populations in developing countries for several reasons. First, contaminated needles and syringes are major problems both for health workers and for environmental safety in many developing countries where there is a high prevalence of HIV and hepatitis B and C [16-18]. Because they obviate the need for needles and syringes, oral vaccines allow less qualified health workers to carry out immunization. Second, the simple logistics of oral vaccines are highly compatible with mass immunization campaigns [19,20]. Lastly, in most societies both adults and children generally prefer an oral vaccine to a parenteral injection.
Despite the attractions of oral vaccines for developing countries, many oral vaccines, both live and non-living, have proven to be less immunogenic or less protective when administered to infants, children or adults living in low socioeconomic conditions in less-developed countries than they are when used in industrialized countries (Table 1). Thus, there is a poorly understood 'intestinal barrier' to successful immunization of people in less developed countries who receive oral vaccines. Here, I review this phenomenon, provide examples and possible explanations, and offer suggestions for establishing the basis of the phenomenon.