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Twelve years after the global polio eradication goal was set, polio cases have declined by more than 95% world-wide. Polio immunization campaigns have been conducted in every endemic country with as many as 470 million children immunized per year. Intense wild poliovirus transmission is now limited to South Asia and sub-Saharan Africa. To achieve eradication at the earliest possible date, immunization campaigns are being intensified in the remaining endemic countries. Major programmatic challenges include reaching vulnerable children in areas with armed conflict and ensuring full financial and political support for the initiative. With global eradication imminent, WHO is preparing for post-eradication issues: containment of polioviruses, certification of eradication, and stopping immunization.

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As the global polio eradication initiative comes ever closer to its goal of terminating of all wild poliovirus transmission, significant challenges remain. Wild poliovirus transmission must be terminated in countries where low-level transmission persists, in large reservoir countries with high population density, and in conflict countries. Eradication can be achieved in these countries with determined and persistent effort, assuming that sufficient resources are mobilised. High quality surveillance needs to be implemented in the remaining polio endemic countries, especially in Africa. Surveillance is necessary in all countries until eradication is certified. A strategy for stopping immunisation after eradication is yet to be defined. A definition of this strategy would address the emergence of feral polioviruses, chronic vaccine virus infection in immunodeficient persons, and containment of laboratory strains. Until immunisation is stopped, high-population immunity should be maintained through uniformly high immunisation coverage with potent vaccines.

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In the 12 years since the global polio eradication goal was set in 1988, the eradication initiative has reached every polio-endemic country. Mass immunization campaigns have reached as many as 450 million children per year and 134 million children in a single day. The number of reported cases has declined from 35,251 to 7088 cases during that time span. Because surveillance has improved the decline in cases occurring is significantly greater. Wild poliovirus transmission is now confined to South Asia and West and Central Africa and the Horn of Africa. Although significant obstacles remain, it appears likely that polio will be eradicated world-wide by the end of the year 2002.

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Disease eradication as a public health strategy was discussed at international meetings in 1997 and 1998. In this article, the ongoing poliomyelitis eradication initiative is examined using the criteria for evaluating candidate diseases for eradication proposed at these meetings, which covered costs and benefits, biological determinants of eradicability (technical feasibility) and societal and political considerations (operational feasibility). The benefits of poliomyelitis eradication are shown to include a substantial investment in health services delivery, the elimination of a major cause of disability, and far-reaching intangible effects, such as establishment of a "culture of prevention". The costs are found to be financial and finite, despite some disturbances to the delivery of other health services. The "technical" feasibility of poliomyelitis eradication is seen in the absence of a non-human reservoir and the presence of both an effective intervention and delivery strategy (oral poliovirus vaccine and national immunization days) and a sensitive and specific diagnostic tool (viral culture of specimens from acute flaccid paralysis cases). The certification of poliomyelitis eradication in the Americas in 1994 and interruption of endemic transmission in the Western Pacific since March 1997 confirm the operational feasibility of this goal. When the humanitarian, economic and consequent benefits of this initiative are measured against the costs, a strong argument is made for eradication as a valuable disease control strategy.

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The global initiative to eradicate poliomyelitis is focusing on a small number of countries in Africa (Angola, Democratic Republic of the Congo, Liberia, Sierra Leone, Somalia, Sudan) and Asia (Afghanistan, Tajikistan), where progress has been hindered by armed conflict. In these countries the disintegration of health systems and difficulties of access are major obstacles to the immunization and surveillance strategies necessary for polio eradication. In such circumstances, eradication requires special endeavours, such as the negotiation of ceasefires and truces and the winning of increased direct involvement by communities. Transmission of poliovirus was interrupted during conflicts in Cambodia, Colombia, El Salvador, Peru, the Philippines, and Sri Lanka. Efforts to achieve eradication in areas of conflict have led to extra health benefits: equity in access to immunization, brought about because every child has to be reached; the revitalization and strengthening of routine immunization services through additional externally provided resources; and the establishment of disease surveillance systems. The goal of polio eradication by the end of 2000 remains attainable if supplementary immunization and surveillance can be accelerated in countries affected by conflict.

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PIP: This article summarizes the WHO-sponsored meeting of virologists, immunologists, and epidemiologists in March 1998 to address the final and controversial stage of the polio eradication initiative. The meeting commissioned Fine and Carneiro's literature review and mathematical model delineating the important gaps in the scientific knowledge and helped define the research agenda of the remaining few years of the initiative. Fine and Carneiro proposed that the possibility of continuing circulation of vaccine-derived polioviruses (VDPV) could not be excluded with absolute certainty. They also argued that VDPV may continue to circulate after use of oral polio vaccine stops and that immunodeficient persons may be a potential reservoir from which VDPV could be reintroduced into the general population. Their work further highlights that high-level enterovirus surveillance will be essential in the years after immunization has stopped.^ieng

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Since the poliomyelitis eradication program began in 1988, the number of poliovirus infected continents and countries have decreased from five to two and from greater than 100 to 53, respectively. A nearly 90% reduction in the incidence of polio has been achieved with a corresponding decrease in virus genomic heterogeneity. Major challenges to eradication remain in south Asia and Africa in those areas with hot and humid climates, high population density, and high birth rates. Of particular concern are countries with ongoing social unrest and poor health infrastructure. With the approaching eradication of polio, post-eradication issues are now being addressed. The World Health Organization (WHO) draft plan for containment of wild polioviruses has been published for comment. Commissions and committees for certification of eradication have been established. Still under discussion is the question of the appropriate strategy for stopping oral polio vaccine (OPV) immunization. Studies are underway to determine whether vaccine-derived polioviruses will continue to circulate after OPV cessation and the potential disease consequences of that circulation.

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Led by an international partnership including Rotary International, the WHO, UNICEF and the Centers for Disease Control and Prevention in the USA, the global initiative to eradicate poliomyelitis has made remarkable progress since its beginning in 1988. The number of polio cases has decreased from an estimated 350,000 cases in 1988 to just over 5,000 reported cases in 1999. Following successful eradication from the WHO Region of the America's, certified as polio-free in 1994, wild poliovirus was last reported from the Western Pacific Region (including China) in 1997, and from the European Region (including all countries of the former Soviet Union) in November 1998. Large parts of Southern and Northern Africa and the Middle East are also polio-free. This success is related to the implementation of annual nationwide supplemental oral poliovaccine (OPV) campaigns in all recently or currently polio-endemic countries, targeting all children under 5 years with two doses of OPV. Progress of the initiative is monitored by special surveillance systems for all cases of acute flaccid paralysis (AFP) in children under 15 years, including virological testing to rule out wild poliovirus infection. The initiative currently focuses on a limited number of remaining endemic countries in South Asia and Africa, including India, Pakistan, Bangladesh, and Nigeria. The initiative is also beginning to be successful in several countries affected by conflict situations, such as Afghanistan, Sudan (South), Somalia, Democratic Republic of the Congo and Angola.

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Ten years after the year 2000 target was set by the World Health Assembly, the global poliomyelitis eradication effort has made significant progress towards that goal. The success of the initiative is built on political commitment within the endemic countries. A partnership of international organizations and donor countries works to support the work of the countries. Interagency coordinating committees are used to ensure that all country needs are met and to avoid duplication of donor effort. Private sector support has greatly expanded the resources available at both the national and international level. At the programmatic level, rapid implementation of surveillance is the key to success, but the difficulty of building effective surveillance programmes is often underestimated. Mass immunization campaigns must be carefully planned with resources mobilized well in advance. Programme strategies should be simple, clear and concise. While improvements in strategy and technology should be continuously sought, changes should be introduced only after careful consideration. Careful consideration should be given in the planning phases of a disease control initiative on how the initiative can be used to support other health initiatives.

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Significant progress is being made towards the global eradication of poliomyelitis by the year 2000. The strategies recommended by the World Health Organization for polio eradication are as follows: maintaining high routine immunization coverage; conducting nationwide mass immunization campaigns; building effective, laboratory-based surveillance for acute flaccid paralysis; and conducting localized immunization campaigns directed at the final reservoirs of virus transmission. Sixty-three countries have conducted nationwide anti-polio immunization campaigns. Three hundred million children were immunized in these campaigns worldwide in 1995. The reported incidence of poliomyelitis has fallen by approximately 80% since the global target was set in 1988, and the geographic range of polio is being restricted. The major challenges for achieving eradication are establishing effective surveillance systems in all countries and mobilizing the resources needed to fully implement the recommended strategies in the 67 countries in which polio remains endemic.

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