The Eradication of Polio — Progress and Challenges
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《新英格兰医药杂志》
Six years after the original 2000 target date for the global eradication of polio, public health workers are encountering several stumbling blocks. Poliovirus circulation persists in countries where the virus is endemic; new outbreaks are occurring in previously polio-free areas, including, most recently, Kenya's first documented wild-type poliovirus infection in 22 years; and complex social challenges stand in the way of public health efforts in some countries.
Since 1988, when the World Health Assembly adopted the goal of eradication, the public health initiative has made extraordinary progress: the disease burden has been reduced by more than 99%, and the number of countries with endemic transmission has been reduced by more than 96%. Yet according to the reports from the global polio surveillance network, as of November 15, a total of 1646 cases had been confirmed this year,1 and the annual incidence has remained steady for the past 4 years (ranging from 784 to 1972 cases per year). This apparent lack of progress in reducing the number of cases in the past few years has resulted in occasional impatience and frustration, even leading some to question the ultimate feasibility of global eradication.
The simple count of cases, however, does not fully reflect either the extent of recent accomplishments or the remaining barriers to eradication.2 The past several years have been a roller-coaster ride in which progress against the virus has repeatedly been followed by setbacks in the form of outbreaks. Nevertheless, only four countries where the virus remains endemic — Nigeria, India, Pakistan, and Afghanistan — account for 93% of the world's cases of poliomyelitis; unlike all other countries, they have never succeeded in interrupting the transmission of wild poliovirus.
A major accomplishment of the polio eradication program is the establishment of a global integrated virologic surveillance network. Because of the rapid mutation rate of poliovirus, molecular surveillance can provide greater insight into its circulation than would be possible with strictly epidemiologic tools. Thanks to continual changes in the genetic material of the virus and the routine molecular characterization of every wild poliovirus isolate in the world, patterns of genetic similarity can be used to help public health officials track individual virus lineages, understand the implied patterns of transmission, and monitor the progress of the eradication efforts.
Genetic sequencing has, for example, confirmed that cases of poliomyelitis reported in Niger and Nepal this year resulted from repeated importation of the virus from neighboring countries where it is endemic, rather than from internal spread. In addition to the 4 countries where the virus is endemic, cases have been reported in 12 other countries this year (see map).3 Two of these countries, Yemen and Indonesia, presumably had the final cases associated with large outbreaks that began in 2005. Others — the Democratic Republic of Congo, Ethiopia, and Bangladesh — have had a small number of cases resulting from importations that occurred this year from Angola, Somalia, and India, respectively. A determination of whether the source of the outbreak is importation or prolonged, undetected endemic transmission is critical for assessing the quality of surveillance and devising appropriately targeted immunization activities.
Countries Reporting Cases of Wild Poliovirus Infection in 2006 and Routes of Viral Spread from Countries with Endemic Disease from 2002 to 2006.
Data for cases in 2006 are from the World Health Organization. Arrows indicate some of the international importation events during this period, as determined by means of poliovirus surveillance and genetic analysis. Shading denotes countries with reported cases of polio in 2006; red represents the four countries where polio is still endemic, and orange represents countries with cases or outbreaks after importation.
Although surveillance and genetic sequencing elucidate the dynamics of poliovirus circulation, they do not tell us why the virus remains endemic in some places. The biologic reason is the same regardless of the location: insufficient immunity in the population to interrupt transmission. In almost all cases, the basic cause remains a failure to vaccinate enough children with enough doses to ensure that they are immune to disease and infection. The reasons for this failure, however, are diverse. In Nigeria, a country with 958 cases (58% of all cases in the world), the deficiencies in vaccination are large, obvious, and geographically localized. In 4 states in northern Nigeria, more than 30% of children are unvaccinated against poliovirus (although this is a substantial improvement, considering that 1 year ago, more than 30% of children in 10 states were unvaccinated). The immunity gap in northern Nigeria is the legacy of a suspension of poliovirus immunization activities in 2003 and 2004, the community's lingering suspicions about the motivations of public officials, and rumored risks associated with the vaccine, such as infection with the human immunodeficiency virus and steroid contamination leading to infertility. Despite the optimism that marked the resumption of immunization activities,4 ongoing political, cultural, and religious objections hinder vaccination efforts, resulting in persistently low immunity in the population and, consequently, a high incidence of disease.
(Figure)
Transmission Electron Micrograph of Poliovirus Type 1.
From Joseph J. Esposito/CDC.
In several countries — particularly Afghanistan and Somalia, but also parts of Pakistan and Ethiopia — political instability or armed conflicts make vaccination logistically difficult and unpredictable at best. Polio can be eliminated from these regions, as it has been in Sudan and was previously in Somalia and Ethiopia, but negotiations for "days of tranquility" (the suspension of hostilities in order to allow for vaccination) will have to remain an integral part of polio eradication activities for the foreseeable future.
(Table)
Cases of Wild Poliovirus Infection in 2006.
In the countries where the virus is still endemic, poliomyelitis largely affects underserved populations. In India, for example, the epicenter of the current circulation of the virus is in western Uttar Pradesh, where nearly 70% of cases occur among members of the minority Muslim population.
Because of persistent viral transmission, polio eradication activities have been intensified in India, and most children in the northern part of the country are now receiving more than 12 doses of vaccine before their second birthday (many more than the number of doses that are recommended for routine vaccinations, but necessary to attain immunity in high-risk areas and to eradicate the virus in some populations). These increased activities have led to fatigue among both the local vaccination teams and the community members who participate. Moreover, a median of 10 reported vaccine doses have been received by persons who have contracted poliomyelitis; this has raised questions about the efficacy of the vaccine. The quality and potency of the vaccine have been tested and shown to be adequate, but various biologic and ecologic factors such as other enteric infections and poor nutrition may be contributing to its reduced effectiveness in inducing immunity in these particular reservoir populations. A perplexing discrepancy remains, however, since other areas of southern Asia — including other parts of India — have similar risk factors for the transmission of the virus because of poor sanitation and crowding and yet are now polio-free.
During the past couple of years, several outbreaks associated with vaccine-derived polioviruses have also challenged the program. One outbreak occurred during the last part of 2005 on the island of Madura in Indonesia; with 46 cases, it is the largest documented outbreak to date. Nearly concurrently, wild poliovirus was also introduced into Madura from Java, resulting in an additional eight cases in a population of only about 3 million. The source of that virus was a wild type 1 poliovirus outbreak occurring in other parts of Indonesia, sparked by an importation of Nigerian poliovirus that had spread through Chad, Sudan, and Saudi Arabia (see map). Both viruses were eliminated from Madura by means of three immunization campaigns. However, the experience offered no major insights into the risk factors for vaccine-derived polio outbreaks, beyond the known contributors of low immunization coverage and the absence of circulating, indigenous wild virus of the same serotype.
A few events have alerted experts in polio eradication to the importance of continuing to monitor the quality of surveillance activities even in the absence of poliovirus. In 2004, when Nigerian polio was spreading across central Africa into Chad and Sudan, other polioviruses that had apparently been circulating unrecognized for several years were detected in these same countries. Presumably, the outbreaks had led to enhanced surveillance activities that permitted the detection of these strains, but the fact that they had not been identified previously was disturbing. A less extreme gap in surveillance was revealed by an outbreak this year in Namibia. This outbreak was caused by a virus that had probably been circulating undetected in Angola since 2004, after an importation from India. Similarly, vaccine-derived poliovirus outbreaks like the Madura outbreak represent the consequences of viruses that have circulated in a population for 1 year or more before being recognized. It is essential to detect all polioviruses earlier if their eradication is to be ensured, but detection is particularly challenging in resource-poor countries.
In the past few years, the initiative to eradicate polio has faced substantial challenges. Large outbreaks associated with spread from the primary global reservoirs in Nigeria and India affected 25 countries and were controlled only after more than 2 years of effort and at least $400 million in supplemental immunization activities. At the same time, the belief that wild type 2 poliovirus was eradicated in 1999 becomes more certain with every passing year, and the fundamental feasibility of eradication when the basic strategies are implemented continues to be demonstrated in all countries. Still, a precise timeline for eradication of the two remaining serotypes (types 1 and 3) remains unpredictable.
Very few people remember whether smallpox eradication was achieved by its target date. Indeed, success virtually ensures that this detail will be forgotten. Such a historical perspective is not yet available to the polio eradication program as it strives to reach the marathon's finish line.
The views expressed in this article are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.
Source Information
Dr. Pallansch is a virologist and Dr. Sandhu is a medical epidemiologist at the National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta.
References
Wild poliovirus weekly update. (Accessed November 22, 2006, at http://www.polioeradication.org/casecount.asp.)
Heymann DL, Aylward RB. Eradicating polio. N Engl J Med 2004;351:1275-1277.
Resurgence of wild poliovirus type 1 transmission and consequences of importation -- 21 countries, 2002-2005. MMWR Morb Mortal Wkly Rep 2006;55:145-150.
Samba E, Nkrumah F, Leke R. Getting polio eradication back on track in Nigeria. N Engl J Med 2004;350:645-646.(Mark A. Pallansch, Ph.D.,)
Since 1988, when the World Health Assembly adopted the goal of eradication, the public health initiative has made extraordinary progress: the disease burden has been reduced by more than 99%, and the number of countries with endemic transmission has been reduced by more than 96%. Yet according to the reports from the global polio surveillance network, as of November 15, a total of 1646 cases had been confirmed this year,1 and the annual incidence has remained steady for the past 4 years (ranging from 784 to 1972 cases per year). This apparent lack of progress in reducing the number of cases in the past few years has resulted in occasional impatience and frustration, even leading some to question the ultimate feasibility of global eradication.
The simple count of cases, however, does not fully reflect either the extent of recent accomplishments or the remaining barriers to eradication.2 The past several years have been a roller-coaster ride in which progress against the virus has repeatedly been followed by setbacks in the form of outbreaks. Nevertheless, only four countries where the virus remains endemic — Nigeria, India, Pakistan, and Afghanistan — account for 93% of the world's cases of poliomyelitis; unlike all other countries, they have never succeeded in interrupting the transmission of wild poliovirus.
A major accomplishment of the polio eradication program is the establishment of a global integrated virologic surveillance network. Because of the rapid mutation rate of poliovirus, molecular surveillance can provide greater insight into its circulation than would be possible with strictly epidemiologic tools. Thanks to continual changes in the genetic material of the virus and the routine molecular characterization of every wild poliovirus isolate in the world, patterns of genetic similarity can be used to help public health officials track individual virus lineages, understand the implied patterns of transmission, and monitor the progress of the eradication efforts.
Genetic sequencing has, for example, confirmed that cases of poliomyelitis reported in Niger and Nepal this year resulted from repeated importation of the virus from neighboring countries where it is endemic, rather than from internal spread. In addition to the 4 countries where the virus is endemic, cases have been reported in 12 other countries this year (see map).3 Two of these countries, Yemen and Indonesia, presumably had the final cases associated with large outbreaks that began in 2005. Others — the Democratic Republic of Congo, Ethiopia, and Bangladesh — have had a small number of cases resulting from importations that occurred this year from Angola, Somalia, and India, respectively. A determination of whether the source of the outbreak is importation or prolonged, undetected endemic transmission is critical for assessing the quality of surveillance and devising appropriately targeted immunization activities.
Countries Reporting Cases of Wild Poliovirus Infection in 2006 and Routes of Viral Spread from Countries with Endemic Disease from 2002 to 2006.
Data for cases in 2006 are from the World Health Organization. Arrows indicate some of the international importation events during this period, as determined by means of poliovirus surveillance and genetic analysis. Shading denotes countries with reported cases of polio in 2006; red represents the four countries where polio is still endemic, and orange represents countries with cases or outbreaks after importation.
Although surveillance and genetic sequencing elucidate the dynamics of poliovirus circulation, they do not tell us why the virus remains endemic in some places. The biologic reason is the same regardless of the location: insufficient immunity in the population to interrupt transmission. In almost all cases, the basic cause remains a failure to vaccinate enough children with enough doses to ensure that they are immune to disease and infection. The reasons for this failure, however, are diverse. In Nigeria, a country with 958 cases (58% of all cases in the world), the deficiencies in vaccination are large, obvious, and geographically localized. In 4 states in northern Nigeria, more than 30% of children are unvaccinated against poliovirus (although this is a substantial improvement, considering that 1 year ago, more than 30% of children in 10 states were unvaccinated). The immunity gap in northern Nigeria is the legacy of a suspension of poliovirus immunization activities in 2003 and 2004, the community's lingering suspicions about the motivations of public officials, and rumored risks associated with the vaccine, such as infection with the human immunodeficiency virus and steroid contamination leading to infertility. Despite the optimism that marked the resumption of immunization activities,4 ongoing political, cultural, and religious objections hinder vaccination efforts, resulting in persistently low immunity in the population and, consequently, a high incidence of disease.
(Figure)
Transmission Electron Micrograph of Poliovirus Type 1.
From Joseph J. Esposito/CDC.
In several countries — particularly Afghanistan and Somalia, but also parts of Pakistan and Ethiopia — political instability or armed conflicts make vaccination logistically difficult and unpredictable at best. Polio can be eliminated from these regions, as it has been in Sudan and was previously in Somalia and Ethiopia, but negotiations for "days of tranquility" (the suspension of hostilities in order to allow for vaccination) will have to remain an integral part of polio eradication activities for the foreseeable future.
(Table)
Cases of Wild Poliovirus Infection in 2006.
In the countries where the virus is still endemic, poliomyelitis largely affects underserved populations. In India, for example, the epicenter of the current circulation of the virus is in western Uttar Pradesh, where nearly 70% of cases occur among members of the minority Muslim population.
Because of persistent viral transmission, polio eradication activities have been intensified in India, and most children in the northern part of the country are now receiving more than 12 doses of vaccine before their second birthday (many more than the number of doses that are recommended for routine vaccinations, but necessary to attain immunity in high-risk areas and to eradicate the virus in some populations). These increased activities have led to fatigue among both the local vaccination teams and the community members who participate. Moreover, a median of 10 reported vaccine doses have been received by persons who have contracted poliomyelitis; this has raised questions about the efficacy of the vaccine. The quality and potency of the vaccine have been tested and shown to be adequate, but various biologic and ecologic factors such as other enteric infections and poor nutrition may be contributing to its reduced effectiveness in inducing immunity in these particular reservoir populations. A perplexing discrepancy remains, however, since other areas of southern Asia — including other parts of India — have similar risk factors for the transmission of the virus because of poor sanitation and crowding and yet are now polio-free.
During the past couple of years, several outbreaks associated with vaccine-derived polioviruses have also challenged the program. One outbreak occurred during the last part of 2005 on the island of Madura in Indonesia; with 46 cases, it is the largest documented outbreak to date. Nearly concurrently, wild poliovirus was also introduced into Madura from Java, resulting in an additional eight cases in a population of only about 3 million. The source of that virus was a wild type 1 poliovirus outbreak occurring in other parts of Indonesia, sparked by an importation of Nigerian poliovirus that had spread through Chad, Sudan, and Saudi Arabia (see map). Both viruses were eliminated from Madura by means of three immunization campaigns. However, the experience offered no major insights into the risk factors for vaccine-derived polio outbreaks, beyond the known contributors of low immunization coverage and the absence of circulating, indigenous wild virus of the same serotype.
A few events have alerted experts in polio eradication to the importance of continuing to monitor the quality of surveillance activities even in the absence of poliovirus. In 2004, when Nigerian polio was spreading across central Africa into Chad and Sudan, other polioviruses that had apparently been circulating unrecognized for several years were detected in these same countries. Presumably, the outbreaks had led to enhanced surveillance activities that permitted the detection of these strains, but the fact that they had not been identified previously was disturbing. A less extreme gap in surveillance was revealed by an outbreak this year in Namibia. This outbreak was caused by a virus that had probably been circulating undetected in Angola since 2004, after an importation from India. Similarly, vaccine-derived poliovirus outbreaks like the Madura outbreak represent the consequences of viruses that have circulated in a population for 1 year or more before being recognized. It is essential to detect all polioviruses earlier if their eradication is to be ensured, but detection is particularly challenging in resource-poor countries.
In the past few years, the initiative to eradicate polio has faced substantial challenges. Large outbreaks associated with spread from the primary global reservoirs in Nigeria and India affected 25 countries and were controlled only after more than 2 years of effort and at least $400 million in supplemental immunization activities. At the same time, the belief that wild type 2 poliovirus was eradicated in 1999 becomes more certain with every passing year, and the fundamental feasibility of eradication when the basic strategies are implemented continues to be demonstrated in all countries. Still, a precise timeline for eradication of the two remaining serotypes (types 1 and 3) remains unpredictable.
Very few people remember whether smallpox eradication was achieved by its target date. Indeed, success virtually ensures that this detail will be forgotten. Such a historical perspective is not yet available to the polio eradication program as it strives to reach the marathon's finish line.
The views expressed in this article are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.
Source Information
Dr. Pallansch is a virologist and Dr. Sandhu is a medical epidemiologist at the National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta.
References
Wild poliovirus weekly update. (Accessed November 22, 2006, at http://www.polioeradication.org/casecount.asp.)
Heymann DL, Aylward RB. Eradicating polio. N Engl J Med 2004;351:1275-1277.
Resurgence of wild poliovirus type 1 transmission and consequences of importation -- 21 countries, 2002-2005. MMWR Morb Mortal Wkly Rep 2006;55:145-150.
Samba E, Nkrumah F, Leke R. Getting polio eradication back on track in Nigeria. N Engl J Med 2004;350:645-646.(Mark A. Pallansch, Ph.D.,)