Is Polio Eradication Realistic?

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Science  12 May 2006:
Vol. 312, Issue 5775, pp. 852-854
DOI: 10.1126/science.1124959

The polio eradication campaign has been of enormous benefit to humankind, reducing the estimated 350,000 polio cases in 1988 to 1,948 in 2005. So far, it has cost $4 billion in international assistance and it has been estimated that eradication (including 3 years of follow-up) could cost another $1.2 billion (1, 2). This is in sharp contrast to the experience with smallpox, for which eradication took only 10 years and international expenditure was only $100 million (1980 value) (3). Furthermore, we believe that global eradication is unlikely to be achieved.

The introduction of the trivalent Sabin oral vaccine (OPV) in 1962 provided a cheap and readily administered vaccine. A regional program of polio eradication was initiated in the Americas in 1985, based on the regular performance of National Immunization Days (NIDs) for all children under 5 years of age and surveillance of acute flaccid paralysis (AFP) of all children under 15 years of age. The last endemic case occurred in Peru in 1991, and regional eradication was certified by a World Health Organization (WHO) committee in 1994. Surveillance and immunization programs have been maintained, using inactivated vaccine (IPV) in North America and OPV elsewhere.

This success led the WHO to launch a global eradication program in 1988, with a target of the year 2000 for the last case. The Western Pacific region initiated its program in 1990 and, by 1997, had interrupted transmission in the region, with WHO certification in 2000.

Eradication programs in the Indian subcontinent, the Middle East, and Africa south of the Sahara were launched in the early or mid-1990s. When the target year of 2000 came, there were still 23 nations reporting cases (see table on page 853), including 9 nations where it was endemic. Intensified efforts to achieve global eradication have been made since then, but in 2005, a total of 1948 cases of poliomyelitis were reported in 16 nations. In four nations it was endemic, and six had large epidemics due to recent importations. There are four main reasons why the eradication of polio has proved so much more difficult than the eradication of smallpox: the high proportion of subclinical cases, vaccine-derived polio, population/political changes, and prolonged duration of the global program.

Subclinical cases. The most important difference between smallpox and polio that affects eradication is that in smallpox there were no subclinical cases; every infectious patient had obvious clinical disease. Smallpox vaccination could be sharply focused to small areas. In India in 1973, active village-by-village search for smallpox cases was specifically conducted and followed immediately by containment vaccination of infected villages. Within 18 months, transmission was interrupted throughout India.

There are 100 to 200 subclinical and therefore “invisible” poliovirus infections for every case of paralysis. Because every infected person excretes the virus, surveillance and containment are impossible, hence the need for NIDs. In India, although repeated rounds of NIDs were carried out between 1995 and 2005, most involving millions of children, cases have continued to be found (see table, right).

Vaccine-derived polio. The difficulties of using OPV for global eradication have recently been described (4). They are vaccine-associated paralytic polio (one in three million vaccines), circulating vaccine-derived polio virus (so far, five outbreak episodes with a total of 50 cases in different geographical areas), and virulent vaccine virus excreted by immune-compromised individuals (19 known cases) (5). A further complication is that vaccine virus may recombine with other enteroviruses; some of these recombinants have caused AFP indistinguishable from polio (6, 7). Outbreaks of paralytic polio resulting from vaccine strain mutations seem to be less neuropathogenic and less transmittable. Certainly, these events require careful study, as their cause and likelihood of occurrence are unclear. In consequence, policies to be followed after termination of polio transmission are uncertain.

Changes in the world population and politics. In 1977, when the last case of smallpox occurred, the world population was 4 billion; now it is 6.2 billion, and almost all of the increase has been in the developing world, where extreme poverty has been recognized (8). Global eradication of infectious diseases requires “once-and-for-all” efforts by all nations; poor nations need increased investment from their limited resources, as well as assistance from outside sources.

Global coordination was easier during the Cold War period as the two superpower blocs strongly supported the smallpox program. Countrywide programs continued successfully during civil war in Nigeria, the India-Pakistan war, and the Ethiopia-Somalia war. The polio eradication program, in contrast, started approximately at the end of the Cold War and as different countries were assuming a greater degree of political independence. This had a direct influence on the polio eradication program, especially in 2003–04 in Africa, when polio vaccination was suspended in northern Nigeria because of a mistaken belief that the vaccine was unsafe (9). This led to extensive polio epidemics which spread to many nations in sub-Saharan Africa and Indonesia.

Duration of global eradication programs. The advantages mentioned above might give the impression that smallpox eradication was easy. It was not so. There were numerous difficulties; such as lack of transport, concealment of smallpox outbreaks by the government, guerrilla wars, chronic shortage of funds, and bureaucratic mismanagement. In 1980, D. A. Henderson, ex-WHO leader of the program, indicated, “… I realize that smallpox eradication was achieved, but just barely achieved. Had the biological and epidemiologic characteristics of the disease, or the world political situation, been even slightly more negative, the effort might have failed” (10).

Why, then, did smallpox eradication succeed? One reason was the short length of the program, at 10 years (11). In 1997, at the Dahlem Workshop on the Eradication of Infectious Diseases, Arita stated, “The duration of an eradication program should not be too long, perhaps in the range of 10–15 years…it is difficult to sustain a high level of enthusiasm throughout the period” (11).

It took 5 years to move from 31 nations with endemic smallpox in 1967 to 8 nations in 1973, and another 5 years to reach zero cases globally in 1977. In the polio program, 18 years have already elapsed. The eradication program succeeded in reducing the number of nations reporting polio from 150 in 1988 to the 23 in 2000 but then slowed to 16 in 2005, despite extensive synchronized campaigns in Africa (see table). By 21 March 2006, 91 total cases were reported to WHO for the year, as compared with 52 from January to March in 2005.

Global Eradication or Control?

The question is, should WHO proceed with its current global eradication program, in view of all the difficulties and uncertainties identified in this paper? Our answer is “No.” The global eradication effort needs to be viewed in the context of the public health situation in Africa (12). In many sub-Saharan nations, mortality rates for children under 5 years old are ∼100 to 200, as compared with 4 in Japan and 8 in the United States. A United Nations group recently concluded that the Millennium Development Goal of 70% reduction of child deaths by 2015 is too high to achieve (13). There are great disparities in routine immunization rates around the world; for example, 90% of children in Europe have been protected against diphtheria, pertussis, and tetanus (DPT) and measles, as compared with about 50% in sub-Saharan Africa (14). Vaccines are not yet available for AIDS and malaria, which represent enormous threats to the developing world.

The monetary figures for international assistance hide the reality that recipient nations, particularly poor nations in the sub-Saharan area and Indian subcontinent have to digest such assistance with extraordinary mobilization of their own health resources. Although international assistance for smallpox eradication was only $100 million for the 10-year operation, recipient nations had to spend $200 million of their own resources. We believe a similar situation has occurred and will continue in future polio eradication efforts. How can areas such as Afghanistan and the sub-Sahara cope with such enormous assistance costs? The unspoken truth is that since 2000, more than 20 poor nations could not cope. We are concerned that international assistance for polio could have negative effects on other public health efforts.

We propose an alternative for consideration by WHO and the international community. We believe the time has come for the global strategy for polio to be shifted from “eradication” to “effective control.” The first priority would be to continue the current emergency measures and limit the spread of polio in Africa, the Middle East, the Indian subcontinent, and Indonesia, as well as other outbreaks which may occur in the future. The recently introduced monovalent OPV might help to achieve this goal (1, 15).

As soon as the annual global number of cases is less than 500 and the number of nations with polio less than 10, all polio eradication elements should become part of the new Global Immunization, Vision, and Strategy (GIVS) program approved by WHO in 2005 (16). There are four major components to the GIVS strategy: “(i) protecting more people in a changing world; (ii) introducing new vaccines and technologies; (iii) integrating immunization, other health interventions, and surveillance in the health systems context; and (iv) immunizing in the context of global interdependence.” Surveillance for AFP should be included in the surveillance of vaccine-preventable diseases. An international vaccine stockpile should be set up with OPV (not IPV) so that if new outbreaks occur, vaccine could rapidly be made available. OPV would continue to be used for routine vaccination in less wealthy countries until 2015, when progress toward the Millennium Development Goals will be evaluated. This strategy would sustain the benefits so far gained by the global polio program and benefit the fight against the many vaccine preventable diseases.

References and Notes

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