INTERRUPTING THE TRANSMISSION OF WILD POLIOVIRUSES WITH VACCINES - IMMUNOLOGICAL CONSIDERATIONS

In 1988 the World Health Assembly set the goal of global poliomyelitis eradication by the year 2000. Substantial progress has been made, and 143 countries reported no poliomyelitis cases associated with the wil d virus in 1993. This article reviews the immunological considerations relevant to interrupting the transmission of wild polioviruses with v accines. Although serum immunity prevents poliomyelitis in the individ ual, it is local immunity that is important in preventing the transmis sion of polioviruses in the community. Natural infection and vaccinati on with oral polioviruses vaccine (OPV) produce local immunity in the intestine and the nasopharynx in about 70-80% of individuals, In contr ast, inactivated poliovirus vaccine (IPV) produces local intestinal im munity in only 20-30% of the individuals. With either vaccine, however , a substantial proportion of the immunized population can transmit th e wild virus. Moreover, although serum immunity is long-lasting, limit ed data suggest that local immunity may not be as persistent. To inter rupt the transmission of wild polioviruses efforts should be made to a chieve and sustain high levels of poliovirus vaccine coverage. Recent outbreaks show that wild poliovirus poses a risk for unimmunized indiv iduals, even when overall coverage levels are high. Delivery of poliov irus vaccine to hard-to-reach populations will be of increasing import ance as countries progress toward the final stages of poliomyelitis er adication. The immunization status of persons from poliomyelitis-free countries should be updated prior to travel to poliomyelitis-endemic a reas.