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Progress Toward Interruption of Wild Poliovirus Transmission --- Worldwide, January 2004--March 2005
In 1988, the World Health Assembly of the World Health Organization (WHO) resolved to eradicate poliomyelitis globally (1). Since then, substantial worldwide progress has been made toward that goal; the number of countries where polio is endemic declined from 125 in 1988 to six by the end of 2003. Further progress in 2004 toward interruption of transmission has continued in the three Asian countries where polio is endemic (Afghanistan, India, and Pakistan) (2,3). However, in 2003, two countries in Africa experienced a resurgence of polio cases; the resurgence continued to spread in 2004 from the Nigeria-Niger endemic reservoir to involve a total of 14 countries that had not reported polio >1 year. Local transmission of wild poliovirus (WPV) has been reestablished in six of these 14 countries, including Sudan, where a major outbreak occurred. This report describes global efforts to eradicate polio during January 2004--March 2005 and outlines remaining challenges to interrupting transmission in countries where polio remains endemic or transmission has been reestablished.
Routine Vaccination Activities
Throughout the world, routine vaccination coverage among infants with 3 doses of oral poliovirus vaccine (OPV3) was estimated at 78% in 2003, the most recent year with fully reported data. Estimated coverage varied substantially among WHO regions*, from 61% in the African Region to 91% in the European Region, with a wide range in estimates for individual countries. In countries where polio is endemic, estimated OPV3 coverage in 2003 was 39% in Nigeria, 51% in Niger, 54% in Afghanistan, 69% in Pakistan, 70% in India, and 98% in Egypt (4).
Supplementary Immunization Activities (SIAs)
To raise population immunity to the level required to interrupt WPV transmission, SIAs to vaccinate children aged <5 years with additional OPV doses (i.e., regardless of routine vaccination history) were conducted in 2004 in 45 countries (six countries where polio is endemic, five with reestablished transmission, seven with virus importations, and 26 polio-free countries at high risk for importation). The 45 countries conducted a total of 171 SIAs§ by using approximately 2.4 billion doses of OPV to vaccinate approximately 372 million children.
In July 2004, polio SIAs were resumed in Kano state in northern Nigeria after their suspension for nearly 12 months because of unfounded concerns regarding vaccine safety that affected campaigns in several northern states (2). Since resumption in Kano state, public acceptance of OPV has increased there and in other northern Nigerian states. In response to the resurgence of polio in Africa and under the auspices of the African Union, synchronized National Immunization Days (NIDs) were conducted twice in late 2004 in 23 western and central African countries and Sudan, reaching approximately 80 million children. However, the November round of NIDs was suspended in Côte d'Ivoire because of renewed armed conflict. Synchronized NIDs were conducted in 21 countries in February and April 2005, with three additional rounds planned for later in 2005.
Acute Flaccid Paralysis (AFP) Surveillance
The quality of AFP surveillance is monitored by two key performance indicators: 1) the rate of nonpolio AFP (NPAFP) cases (i.e., those not caused by WPV), which has a target of one or more cases per 100,000 children aged <15 years; and 2) the proportion of persons with AFP with adequate stool specimens, which has a target of >80%¶. All specimens are processed in a polio laboratory accredited by WHO and annually reviewed; all WPV isolates undergo genomic sequencing in specialized reference laboratories.
The overall sensitivity of AFP surveillance, as measured by the two indicators, was maintained at or near target levels in all WHO regions in 2004 (Table); the global average NPAFP rate increased from 1.9 in 2003 to 2.3 in 2004 (ranging in 2004 from 1.1 in the European Region to 3.0 in the African Region). The proportion of persons with AFP with adequate specimens remained at 86% globally. However, when averaging global, regional, or national indicators, suboptimal performance quality indicators in smaller areas might be masked. For example, 136 (23%) of the 594 districts in India in 2004 did not meet the percentage target for adequate specimen collection**; in Nigeria, 304 (40%) of the 774 local governments did not meet the target.The increase in NPAFP rates in regions where polio is endemic was largely the result of increased AFP reporting from three countries: India (NPAFP rate increased from 2.0 per 100,000 children aged <15 years in 2003 to 3.0 in 2004), Nigeria (6.0 to 8.2), and Pakistan (3.0 to 3.5). These rate increases followed efforts to improve the sensitivity of field AFP surveillance; in India and Pakistan, these efforts were concurrent with decreasing intensity of virus transmission. All six countries where polio is endemic and four (Burkina Faso, Côte d'Ivoire, Mali, and Sudan) of the six countries with reestablished transmission met target surveillance indicators at the national level.
Detailed analyses of genetic sequencing information in 2004 revealed the persistence of surveillance quality gaps in many countries with endemic or reestablished transmission, which was confirmed by analysis of the primary indicators of surveillance quality at the subnational level. Both type 1 and type 3 WPV lineages were isolated in Sudan that genetically indicated circulation had been undetected for at least 3 years in the Chad/Sudan epidemiologic block. Similarly, genetic evidence of surveillance gaps and undetected transmission of WPV type 1 and type 3 for considerable periods was identified in Nigeria and other western African countries and to a lesser extent in Pakistan and Afghanistan. Indicators of suboptimal surveillance quality have been identified in subnational areas of these countries. Surveillance gaps in several countries were identified generally in areas affected by security problems (e.g., southern Sudan, Kandahar region of Afghanistan, and southern Punjab province in Pakistan), which permitted limited access for vaccination and surveillance activities. Despite the 2004 polio epidemic in the African Region, NPAFP rates in the majority of western African countries either did not increase or declined.
Despite a decrease in Afghanistan, India, and Pakistan from 336 reported polio cases in 2003 to 193 to 2004, the number of polio cases reported globally increased from 784 in 2003 to 1,266 in 2004. This was the result of the increase in transmission in Nigeria from 355 polio cases in 2003 to 792 reported cases in 2004 and extensive subsequent transmission in other African countries, including an outbreak in Sudan in late 2004 (6) that continues (Table). The Sudan outbreak (126 cases) subsequently led to virus importations into Ethiopia and Saudi Arabia.
In 2004, India reported its lowest number of cases ever (136), with transmission primarily limited to key districts in western Uttar Pradesh and Bihar (Figure 1). The campaign in India continued to focus on vaccinating the last remaining pockets of unvaccinated or undervaccinated children. District and subdistrict plans were revised in 2004 to better reach underserved communities in areas with populations at high risk and to vaccinate more children moving through busy transit points, such as bus or railway terminals.
In Pakistan, long periods occurred without WPV reported in either North-West Frontier (5 months) or Balochistan provinces (13 months), suggesting that WPV transmission is limited primarily to southern Punjab and Sindh provinces. However, WPV type 1 was recently isolated again in North-West Frontier and Balochistan provinces; this finding and the observed subnational surveillance gaps indicate that low-level transmission might still be widespread.
In Egypt, only one WPV type 1 confirmed case of polio was reported in 2004 (Figure 2); however, sampling of sewage indicates persistent, low-level transmission of two separate lineages of WPV type 1 across the country, particularly in Cairo/Giza (lower Egypt) and Minya and Asiut governorates (upper Egypt) (7). WPV type 3 has not been reported since December 2000.
During 2004, Nigeria experienced a national polio outbreak that expanded beyond the northern states, where endemic transmission has occurred uninterrupted. Nigeria accounted for 62% of the global 2004 case burden (792 of 1,266 cases). Across the African continent and Saudi Arabia, 14 previously polio-free countries had poliovirus importations during 2003--2004 that spread from the Nigeria-Niger endemic reservoir, most recently including Ethiopia. Indigenous WPV transmission (i.e., transmission of an imported virus for >6 months) has been reestablished in six previously polio-free countries: Burkina Faso, the Central African Republic, Chad, Côte d'Ivoire, Mali, and Sudan.
Reported by: Dept of Immunization, Vaccines, and Biologicals, World Health Organization, Geneva, Switzerland. Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Global Immunization Div, National Immunization Program, CDC.
Substantial progress toward polio eradication was achieved during 2004 in India, Pakistan, Afghanistan, and Egypt, where the lowest-ever levels of confirmed polio cases were reported during the second half of 2004, which is the peak season for WPV transmission. In addition, during January--March 2005, the number of cases remained low. The focus currently is on implementing high-quality vaccination campaigns in areas of these countries with populations at highest risk, in an attempt to interrupt polio transmission by mid-2005.
No cases caused by indigenous WPV type 2 have been reported in the world since September 1999. Epidemiologic and virologic evidence suggest that WPV type 3 might no longer circulate in Egypt and, since the second half of 2004, in most areas in northern India where polio is endemic. Because of the probable absence of WPV type 3, monovalent oral polio vaccine type 1 (mOPV1) will be used as an additional tool to interrupt the last chains of WPV type 1 transmission in these countries. This vaccine elicits a higher population immune response per dose against WPV type 1 than the trivalent OPV and will be used in high-transmission areas of India during April--May 2005 and nationwide in Egypt in May and in selected SIAs in the future.
Synchronized vaccination campaigns in 2000 and 2001 succeeded in halting WPV transmission in all western and central African countries except Nigeria and Niger, where endemic transmission has continued. The resurgence of polio in Africa that occurred in 2003--2004 followed a decrease in the number, extent, and quality of SIAs in many countries. To counteract the resurgence of polio, the quality of vaccination campaigns must be improved, particularly in Nigeria and Niger. Countries in Africa will continue to synchronize their polio NIDs during 2005--2006. Progress in Africa could be substantial if 1) NID quality improves and is sustained, 2) synchronized NIDs cover all countries, and 3) remaining gaps in surveillance are closed.
The genetic evidence of surveillance gaps, most pronounced in Chad and Sudan, where low-intensity WPV type 1 and type 3 circulation went undetected for prolonged periods, is of substantial concern because of its implications for the final precertification phase of polio eradication (CDC, unpublished data, 2005). All countries must closely monitor surveillance quality by using performance indicators for subnational areas to detect and correct any remaining problems affecting surveillance sensitivity; this will ensure rapid detection of circulating virus or importation and timely response.
In areas of both Asia and Africa, the greatest risk to polio eradication is a continued failure to vaccinate all children at high risk, because of limited supervision, cultural or geographic hindrances, or conflict. Certain population subgroups continue to be missed with vaccination because of their minority status and limited access to health services. Full commitment of governmental, traditional, and religious leaders at all levels is critical to ensure all children are vaccinated during polio immunization rounds, with particular effort to increase detailed planning and supervision in areas with populations at high risk. Conflicts in Côte d'Ivoire and Sudan pose added challenges to reaching the eradication goal.
In January 2004, the Global Polio Eradication Initiative, in consultation with ministers of health of countries where polio is endemic, discussed a revised target of complete global interruption of poliovirus transmission by the end of 2005. With sufficient financing, political commitment, and motivation of health workers, global eradication is nearly in reach. However, the eradication partnership might be critically challenged if the end of 2005 finds 1) continued poliovirus transmission in the remaining high-risk districts in Afghanistan, India, and Pakistan; 2) delay in the control and interruption of WPV transmission in currently affected African countries; or 3) continued expansion of the epidemic in Africa to additional countries. The initiative will be aided in the final push by selective use of mOPV1 and improved efforts to reach every child in underserved population subgroups. In view of the investments already made in polio eradication, concerted efforts in 2005 by governments, health workers, volunteers, and local and international polio partners are imperative to reach the goal of interrupting poliovirus transmission.
* WHO has six designated regions: African (46 member states and areas), American (35), Eastern Mediterranean (22), European (52), Southeast Asian (11), and Western Pacific (27).
Scheduled SIAs consist of 1) national immunization days (NIDs), which are nationwide mass campaigns during a limited number of days in which doses of OPV are administered to children (usually aged <5 years) regardless of previous vaccination history, with an interval of 4--6 weeks between doses, and 2) subnational immunization days (SNIDs), which are campaigns similar to NIDs but confined to certain parts of the country. Mopping-up SIAs are more intensified campaigns conducted in areas of poliovirus transmission.
§ 115 NIDs, 42 SNIDs, and 14 mopping-up activities.
¶ Two stool specimens collected >24 hours apart within 14 days of paralysis onset and received in the laboratory in acceptable condition.
** For areas with >100,000 children aged <15 years. The genetic sequence of the complete VP1 coding region is determined by using automated dye-labeled cycle sequencing procedures described previously (5) and comparing the resulting sequences with a database of all recent poliovirus isolates. The comparisons are summarized through phylogenetic analysis.
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Date last reviewed: 4/28/2005