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Progress Toward Global Eradication of Poliomyelitis, 2001
From the initiation of the global poliomyelitis eradication initiative in 1988 (1) through 2001, the number of countries where polio is endemic decreased from 125 to 10, and the number of reported polio cases decreased by >99% from an estimated 350,000 to <1,000. Wild type 2 poliovirus has not been detected worldwide since October 1999 (2). The American and Western Pacific Regions of the World Health Organization (WHO) have been certified free of indigenous wild poliovirus (3,4). Current challenges to global polio eradication efforts include ongoing intense transmission in northern India, continued importations of wild poliovirus into polio-free areas, and the detection of circulating vaccine-derived poliovirus (cVDPV). This report summarizes global progress in polio eradication during 2001 and the current status of the initiative.
Implementation of Polio Eradication Strategies
In 2000, reported global vaccination coverage with 3 doses of oral poliovirus vaccine (OPV3) among children aged <12 months was 82%. Routine coverage varies across WHO regions. The African Region (AFR) reported the lowest OPV3 coverage (55% in 2000). In most countries or areas where polio remained endemic in 2001 (Figure 1), OPV3 coverage in 2000 was <50%.
All countries where polio is endemic and many countries where polio was recently endemic continued to conduct supplemental immunization activities (SIAs) during 2001. Approximately 575 million children in 94 countries received an estimated 2 billion doses of OPV during 300 rounds of National Immunization Days* (NIDs), sub-NIDs, or mop-up activities. All countries used house-to-house vaccination in part or all of the SIA target areas as the primary means to reach the highest possible coverage of children aged <5 years.
The quality of acute flaccid paralysis (AFP) surveillance to indicate reliably the presence or absence of wild poliovirus has improved (Table 1). All regions have reached the WHO-required annual nonpolio AFP detection rate of at least one per 100,000 persons aged <15 years (5). Progress in AFP reporting during 2000--2001 was notable in AFR, especially in Nigeria and Ethiopia. Similar progress was made in improving the reliability of virological surveillance through collection of adequate stool specimens. In 2001, all WHO regions except AFR reached the certification requirement of collecting two adequate stool specimens from at least 80% of AFP cases.
The WHO-accredited global network of 147 collaborating laboratories supports eradication strategies. Improvements in the efficiency and logistics of stool specimen handling and transport between collaborating laboratories in many countries enabled the network to reduce reporting delay between onset of paralysis and receipt of final laboratory results.
Impact on Poliovirus Transmission
In 2001, 537 confirmed polio cases (as of March 10, 2002) were reported; 473 (88%) were laboratory-confirmed (Table 1). In 2000, a total of 2,971 cases were reported, of which 719 (24%) were laboratory-confirmed. Because several AFP cases with onset of paralysis in 2001 are still pending final classification in all regions, the final number of confirmed cases for 2001 will be higher, but will probably remain below 1,000. Ten countries documented indigenous transmission of wild poliovirus during 2001, a decrease from 20 countries in 2000. Democratic Republic of Congo, Congo, Chad, Central African Republic, Benin, Ivory Coast, Ghana, Bangladesh, Nepal, and Iraq reported indigenous poliovirus transmission in 2000 but not in 2001.
Three densely populated countries (India, Pakistan, and Nigeria) represent major poliovirus reservoirs. Following a rapid decrease in the extent of wild virus transmission in India during 1997--2000 (6), wild poliovirus types 1 and 3 continued to be transmitted during 2001, mainly in the northern Indian states of Uttar Pradesh and Bihar. The number of laboratory-confirmed cases reported from India did not decrease from 2000 (265 cases) to 2001 (268 cases as of March 10, 2002). However, genetic analysis indicates that three poliovirus lineages circulated in India in 2001, compared with eight in 2000. The intensity and geographic extent of virus transmission in the Pakistan-Afghanistan epidemiological block also decreased from 2000 to 2001. In Pakistan, large population centers in Punjab and Sindh provinces became virus-free, and genetic evidence indicates decreasing virus biodiversity in remaining areas. However, multiple virus foci remained during 2001 in all provinces of Pakistan, with particularly intense transmission in the Northwest Frontier province. In Afghanistan during 2000--2001, virus transmission outside the two areas contiguous to Pakistan appears to have ceased. In 2000, endemic foci were found throughout Nigeria; however, during 2001, transmission became more localized in northern states of Nigeria and adjoining parts of Niger.
During 2001, three polio-free countries detected importations of wild poliovirus: Bulgaria (7), Georgia, and Zambia. The origin of a wild virus detected in Mauritania in 2001 is uncertain, but the virus probably was imported from areas where polio is endemic. Virus importations were detected rapidly and triggered immediate and appropriate large-scale surveillance and vaccination responses that prevented spread of the imported virus.
Polio cases attributed to cVDPV type 1 were found in Haiti and the Dominican Republic during 2000--2001 and in the Philippines during 2001 (8). The virus in both episodes showed >2% genetic sequence difference from the parent Sabin virus (VP1 region of genome) and probably circulated for >2 years before being detected. Public health investigations suggest that low vaccination coverage is allowing cVDPVs to circulate and revert to wild-type characteristics. Mass vaccination campaigns with OPV appear to have interrupted cVDPV circulation in Hispaniola and are underway in the Philippines. The global polio laboratory network established additional procedures to screen for cVDPV. Regional reference laboratories now routinely conduct intratypic differentiation of all poliovirus isolates using both antigenic and molecular tests. Isolates with discrepant results are referred for genetic sequencing studies. Retrospective analysis of approximately 2000 Sabin isolates from AFP cases detected during 1998--2000 has not revealed additional cVDPV.
Preparations for Posteradication Activities
When transmission of wild poliovirus is interrupted globally, wild poliovirus will remain only in diagnostic, research, and vaccine production laboratories. The goal of laboratory containment is to minimize the risk for inadvertent reintroduction of wild poliovirus from a laboratory into human circulation. The Global Plan of Action for Laboratory Containment of Wild Poliovirus (9) requires countries to identify all laboratories storing wild poliovirus or potentially infectious materials to ensure proper handling or disposal under appropriate biosafety conditions. This process has begun in 110 countries.
Planning for postcertification vaccination policy has three main objectives: 1) to develop contingency plans for reintroduction of poliovirus after certification, 2) to prevent cVDPVs from circulating and causing outbreaks, and 3) to broaden the knowledge base to support building a global consensus about the safest and most effective strategy for eventually stopping oral polio vaccination. WHO, CDC, and other partners are coordinating a research agenda to address these issues systematically (10).
Reported by: Vaccines and Biologicals Dept, World Health Organization, Geneva, Switzerland. Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Global Immunization Div, National Immunization Program, CDC.
Acceleration of polio eradication activities during 2001 in countries where polio is endemic resulted in a reduction of both the extent of transmission and diversity of virus lineages. Eradication activities accelerated even in conflict-affected countries. During September 2001, a total of 35 million children received vaccine in Afghanistan and Pakistan, and unprecedented political commitment supporting national polio efforts was given in central and western Africa and India, with heads of state launching NIDs.
Several challenges remain in reaching the global polio eradication goal. The combination of large, dense populations, poor sanitation, and low routine vaccination coverage in Nigeria, Pakistan, and northern India might result in sustained transmission in these countries into 2003. Delays in interrupting transmission also could occur in less populated, conflict-affected countries if conflicts intensify or if access to children in these countries cannot be sustained. In areas where routine OPV coverage is low, interrupting wild poliovirus transmission requires high-quality, supplementary vaccination. National polio eradication efforts require both continued vigilance by the health sector alone and increased political commitment and multisector mobilization at all administrative levels.
The global eradication program has identified and defined the key risks to eradication in each country and is implementing country-specific response plans. Following the example of India, Technical Advisory Groups (TAGs) of experts have been formed for most countries where polio remains endemic, with support from the global polio partnership. TAGs regularly review each country's program status and advise on optimal strategy implementation. AFP surveillance systems have improved dramatically through placement of additional polio field staff. Expert groups exist in almost every country to improve the accuracy of final AFP case classification. Genetic sequencing data are used more systematically to identify main poliovirus circulation areas. The ability of SIAs to reach every child has improved through house-to-house vaccination with emphasis on social mobilization and information activities.
Interagency coordination committees (ICCs) at regional, national, and subnational levels facilitate coordination of partner and multisector support for the polio program and focus on fundraising at the regional level. ICCs created for polio eradication are being used increasingly to coordinate partner input to strengthen vaccination systems including routine vaccination services and vaccine-preventable disease surveillance. During 2000--2001, polio funds were used to purchase approximately 700 vehicles for vaccination and surveillance. An estimated 30% of the cold chain in sub-Saharan Africa has been refurbished through polio eradication funds. Approximately 80% of the nearly 2,000 health-care staff recruited with polio funds are involved in planning and implementing routine vaccination services.
Access to all children, continued political commitment, and assurance of uninterrupted funding will be necessary to interrupt wild poliovirus transmission in countries where polio remains endemic.
* Nationwide mass campaigns over a short period (days to weeks), in which 2 doses of OPV are administered to all children (usually aged <5 years), regardless of vaccination history, with an interval of 4--6 weeks between doses.
Focal mass campaign in high-risk areas over a short period (days to weeks) in which 2 doses of OPV are administered during house-to-house visits to all children (usually aged <5 years), regardless of previous vaccination history, with an interval of 4--6 weeks between doses.
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