Strategies for the Global Eradication of Poliomyelitis by the Year 2000

The initiative for the global eradication of poliomyelitis is being coordinated by the Expanded Programme on Immunization (EPI), established by the World Health Assembly in 1974. The initial goal of the EPI was to provide immunization services by 1990 to all the children of the world during the first year of life against diphtheria, tetanus, pertussis, measles, tuberculosis, and poliomyelitis. The immunization of women of childbearing age to protect newborns against neonatal tetanus was also established as a priority.

In 1974, immunization coverage in developing countries was estimated to be less than 5 percent with vaccines distributed by the EPI, except for slightly higher coverage with bacille Calmette–Guérin (BCG) vaccine. In the ensuing years, the immunization programs were strengthened, and the EPI grew into a broad-based coalition of national programs, United Nations organizations, multilateral and bilateral development agencies, and voluntary groups. According to data reported to the EPI as of August 1991, 83 percent of children worldwide were receiving three doses of diphtheria–pertussis–tetanus (DPT) vaccine and 85 percent were receiving three doses of trivalent oral poliovirus vaccine (TOPV) in the first year of life (Fig. 1). Coverage with BCG vaccine was 90 percent, and with measles vaccine, 80 percent. The immunization of women of childbearing age with tetanus toxoid still lags behind. At present, the coverage of pregnant women with two or more doses of tetanus toxoid is only 34 percent worldwide.

With the attainment of these levels of coverage, the priorities² of the EPI for the 1990s have become (1) achieving and sustaining in all countries full immunization coverage with all the antigens used by the EPI; (2) controlling the targeted diseases by such measures as eradicating poliomyelitis globally by the year 2000, reducing the number of cases of measles by 90 percent of the number before the vaccine became available, and eliminating neonatal tetanus by 1995; (3) improving surveillance to provide an accurate assessment of the progress of the program; (4) introducing within routine immunization services new or improved vaccines as these become available for public health use; (5) promoting other practices in primary health care that are appropriate for the delivery system of the EPI and the target population; and (6) carrying out research and development in support of the above.

Poliomyelitis is the first disease whose eradication is a goal of the EPI.³ Poliomyelitis is inherently more difficult to eradicate than smallpox. Among the epidemiologic characteristics in which the two differ are the asymptomatic illness that is characteristic of most poliovirus infections and the ability of the poliovirus to spread by enteric transmission, both of which make the identification and containment of cases more difficult. In contrast, smallpox was clinically obvious and eradication quite easy to confirm. Differences between the vaccines are also important. Smallpox vaccine is heat stable, one dose is required for protection lasting several years, and vaccination leaves a readily visible scar. TOPV loses substantial potency after one day at 37°C, and multiple doses are required for full protection. Another difference is that properly administered smallpox vaccine has been a highly effective immunogen, whereas seroconversion rates after one to four doses of TOPV have been suboptimal in developing countries.⁴ Confirming that poliovirus has stopped being transmitted will require far more sophisticated tests and facilities. In spite of these and other differences, the eradication of smallpox provides the EPI with a model of success. The most promising evidence that poliomyelitis can be eradicated has come from the Americas.

In May 1985, the Pan American Health Organization adopted the goal of eradicating the indigenous transmission of wild-type poliovirus in the Americas by 1990.⁵ This goal was seen as a springboard for strengthening the entire EPI and as a vehicle for further improving the overall health care infrastructure. The major impediments to poliomyelitis eradication were considered to be a lack of sustained political, financial, and social commitment; managerial constraints; inadequacy of epidemiologic surveillance; and less-than-ideal efficacy and stability of vaccines.

Two strategies have been shown to be critical to the success achieved so far. The first is the maintenance of high immunization coverage with TOPV, accomplished through strong emphasis on routine administration of this vaccine by the permanent health services and through the observance of national "immunization days" twice a year one month apart. During these days all children under five years old receive one dose of TOPV irrespective of their immunization status; other antigens used by the EPI are also administered. The second strategy is to emphasize epidemiologic surveillance. The development of effective surveillance systems has permitted strategies for control to evolve in response to changes in needs. At present, 80 percent of the nearly 20,000 health facilities in the regional system of poliomyelitis surveillance are reporting weekly on the presence or absence of cases of flaccid paralysis. About 80 percent of the patients with reported cases are being seen by trained epidemiologists, who collect two stool specimens from each patient and one specimen from each of five contacts. During the past 12 months, stool samples from 1860 patients with flaccid paralysis were submitted to a network of eight laboratories for poliovirus isolation. The maintenance of such a system requires trained staff, a reliable transport system, and laboratories with reliable diagnostic capabilities.

Uniform case definitions have been adopted by all countries, and effective disease-monitoring systems have been developed to identify acute flaccid paralysis and thus allow prompt intervention. Such intervention includes case investigation and house-to-house immunization with TOPV — so-called mopping-up immunization — in areas where wild-type poliovirus is being transmitted.

In the Americas, the number of stool specimens positive for the wild virus has steadily declined in spite of increased surveillance. Thirty-eight cases due to wild-type poliovirus were reported in 1988, 24 in 1989, and 15 in 1990.^6,7 In the first 38 weeks of 1991, only seven cases of poliomyelitis were confirmed — six in Colombia and one in Peru.⁸ Since April 16, no cases have been detected in spite of an intensive search of all areas considered at risk because of a low rate of vaccination coverage or poor surveillance. The response to the cases in Colombia included house-to-house visits of nearly 1 million households along the coastal areas of the country, with immunization of nearly 1 million children under five years of age. A reward of $100 will be given to any person in the Americas who identifies a case of poliomyelitis due to wild-type poliovirus, and an international certification commission has been formed that will eventually certify that poliovirus transmission has ceased in the Americas.

Relying heavily on the experience in the Americas, the World Health Assembly in 1988 drafted and endorsed a Plan of Action for the Global Eradication of Poliomyelitis by the Year 2000. ¹ The plan identified needs for immunization coverage, surveillance, investigation of outbreaks and their control, and quality control for vaccines (including an effective cold chain to ensure the delivery of potent vaccine). Further operational needs addressed in the plan include laboratory services, training, social mobilization, and rehabilitation. To help set priorities, the plan identified four stages of eradication. Countries or areas in stage A are considered to be free of poliovirus. These countries have a reliable reporting system, have reported no indigenous cases of poliomyelitis for at least the previous three years, and have rates of immunization coverage of at least 80 percent with a full course of vaccine among children reaching their first birthday. Stage B countries or areas have immunization coverage exceeding 50 percent and report fewer than 10 cases of poliomyelitis per year. Stage C countries or areas have immunization coverage exceeding 50 percent and report 10 or more cases of poliomyelitis per year. Stage D countries or areas have immunization coverage of 50 percent or less or an unknown rate of coverage, or report 10 or more cases of poliomyelitis per year or have an unknown number of cases.

As of the end of 1990, most of the world's population lived in areas considered to be in stage C (68 percent). Twenty-four percent were living in areas considered to be in stage A or B, and 8 percent were living in areas considered to be in stage D (Fig. 2). On the basis of these levels of global coverage, the EPI estimates that in 1990 immunization prevented a total of 442,000 cases of paralytic poliomyelitis. Despite this success, an estimated 116,000 cases occurred worldwide during 1990.

During the coming five years, efforts at eradication will be focused on establishing and extending poliomyelitis-free geographic zones. TOPV is the recommended vaccine, although inactivated poliovirus vaccine (IPV) has achieved successful control in certain European countries, Iceland, and some Canadian provinces. The strong preference for TOPV is based on its low cost, ease of administration, superiority in conferring intestinal immunity, and ability to infect household and community contacts, thus extending vaccine coverage.⁹ In countries with endemic poliomyelitis, TOPV should be administered at birth and at 6, 10, and 14 weeks of age. In a number of areas, interruption of the transmission of wild-type poliovirus will require higher levels of coverage than can be expected with routine immunization. Supplemental strategies such as the use of national or local immunization days, outbreak control, and the identification of areas that are at high risk or underserved may be required. As in the Americas, it is recommended that such strategies include, whenever possible, immunization with all vaccines used by the EPI (to be given to persons in eligible age groups who have not received them previously) and ensure that the permanent health care infrastructure is strengthened.

In countries where effective surveillance identifies fewer than 50 cases of poliomyelitis per year, efforts are being made to respond immediately to suspected cases. These efforts include full characterization of the illness and its causal agent. It is recommended that a dose of TOPV be given to children under five years of age living in the zone that is epidemiologically at risk, ideally on a house-to-house basis, regardless of their immunization status. Immunization should be repeated four to six weeks later.

Laboratory confirmation of the clinical diagnosis of poliomyelitis will focus on the isolation of the virus from stool specimens and its characterization. Standardized laboratory methods are recommended in the Manual for the Virological Investigation of Poliomyelitis ¹⁰ of the World Health Organization. Serologic evaluation is not recommended to confirm a diagnosis of poliomyelitis, although it may have a role in assessing seroconversion after the administration of vaccine and in investigating outbreaks.¹¹

In 1988, the World Health Organization estimated the total cost of the global eradication effort from 1989 to 2000 to be $155 million more than the amount needed for routine activities of the EPI.³ In the light of the experience in the Americas, these estimates are being revised, and depending on the quantities of polio vaccine required in excess of the routinely scheduled doses, the total cost may be as much as 10-fold higher. It is important to add the qualification that the estimates do not include the costs of sustaining high levels of immunization coverage in developing countries. These latter costs total some $1 billion per year and are borne in large part by the developing countries themselves, particularly the costs of personnel. External donors provide approximately $300 million per year, mainly for vaccines, supplies, and equipment. Under the international tendering system used by the United Nations Children's Fund (UNICEF), all vaccines necessary for full immunization of a mother and her child against diseases targeted by the EPI (measles vaccine, BCG vaccine, three doses each of TOPV and DTP, and tetanus toxoid for women of childbearing age) can be purchased for less than $1.

Eradication will depend heavily on the application of vaccines and immunization strategies that already exist to achieve full immunization of the world's children against poliomyelitis. Yet, operational research to improve immunization strategies, epidemiologic research to improve surveillance techniques, and basic research to improve vaccine performance will also contribute to the eradication effort.¹²

The eradication of smallpox by a campaign led by the World Health Organization that culminated in 1977 represents one of humanity's greatest achievements in health.³⁰ The cost was about $300 million; the resultant savings are estimated to be $1 billion per year. The task of eradicating poliomyelitis is technically more formidable, and the final financial cost may well be greater than that of eradicating smallpox. Nevertheless, our goal appears to be achievable with existing vaccines and approaches. The future savings are estimated to be $114 million per year in the United States alone. The eradication of poliomyelitis by the year 2000 would be a magnificent gift from the 20th century to future generations of children.