The Mechanism of Poliovirus Eradication

Systematic international efforts to eradicate polio from the developing world began in the Americas in 1985, when the Pan American Health Organization (PAHO) declared a target date of 1990 for the eradication of polio throughout the Americas. Surveillance for wild polioviruses has two arms: (i) acute flaccid paralysis (AFP) case investigations and (ii) virologic studies of polioviruses obtained from clinical specimens. AFP surveillance by itself is neither highly specific nor highly sensitive for detecting individual wild poliovirus infections. Poliovirus eradication has achieved the elimination of individual lineages (equivalent to chains of transmission), different genotypes (groups of related lineages sharing >85% nucleotide sequence identity), and probably wild poliovirus type 2. Polio cases associated with these importations have revealed pockets of unimmunized children in the new host areas, prompting local immunization responses. However, by far the most effective response is to eliminate the source reservoirs. Various supplementary approaches are implemented to monitor ongoing laboratory performance. A serious challenge to the integrity of poliovirus surveillance data is the occurrence of poliovirus contamination of cultures. Recently, polio outbreaks associated with circulating vaccine-derived poliovirus (cVDPV) have been recognized in three different parts of the world. The occurrence of iVDPVs and cVDPVs appears to be rare. Moreover, most chronic poliovirus excretors in developed countries spontaneously stop shedding or die of complications from their immunodeficiency. As an increasing number of highly developed countries have switched to inactivated polio vaccine (IPV), the chances for the occurrence of new iVDPV infections have decreased.