As the worldwide polio eradication campaign enters its final stages, a debate is growing among scientists about when and how immunization against poliomyelitis should be stopped after the disease has been eradicated. The key issues of the debate are in the latest issue of the Bulletin of the World Health Organization, dated March 2000.

The largest disease eradication initiative ever launched is now moving towards its final phase. "This collective human endeavour…will take its rightful place as one of society's most significant achievements", an editorial in the journal says.

Since 1988, when WHO's World Health Assembly adopted a resolution to eradicate polio by the year 2000, the wild poliovirus has been eliminated from three of the five continents.

Cases worldwide have fallen by over 95% to an estimated 20 000 last year. Most of these occur in Bangladesh, Ethiopia, India, Nigeria and Pakistan. There has been much progress in these countries, as well as in others which pose enormous challenges.

Many of the countries where war and conflict have contributed to the disintegration of health systems are managing to immunize their children against polio. Last year, health workers in the Democratic Republic of the Congo immunized over 8 million children during special days of tranquillity brokered by the United Nations. Angola, Afghanistan, Somalia and Southern Sudan also reached millions of children during National Immunization Days.

This progress strengthens hopes that polio will be declared eradicated by the campaign's "certification" deadline of 2005. However, polio-related problems are certain to continue beyond that date – including the possible re-emergence of the disease years later. This is a potential problem with any eradication effort.

In the case of polio, it has long been known that polioviruses in the vaccine can be transmitted between vaccinated individuals and their contacts. A question troubling some researchers is the extent to which these viruses could persist in people after all vaccination has stopped. Studies in several countries, however, have shown no evidence of vaccine virus circulating after immunization stopped. Despite these studies, some experts express concern.

"The susceptibility of the population to poliovirus will gradually increase following the discontinuation of the vaccination – eventually to an extent when widespread epidemic transmission could take place," the three authors of one paper in the journal write.

"The most important issue is to safeguard the population, so that when and how to stop the administration of polio vaccines may be the most critical and potentially far-reaching decision of the entire eradication initiative," they write. The authors are Dr D.J. Wood, of the National Institute for Biological Standards and Control, London, UK, Dr R.W. Sutter, of the Vaccine Preventable Disease Eradication Division at the Centers for Disease Control and Prevention, and Dr W.R. Dowdle, of the Task Force for Child Survival and Development, both in Atlanta, Georgia.

Dr Barry Schoub, director of the National Institute for Virology in Sandringham, South Africa, writes: "The ultimate indirect benefit from the eradication of a vaccine-preventable disease is the ability to make the decision to stop vaccinating and reap the resultant cost and safety gains."

"However, the spectre of a vulnerable population, progressively increasing in its vulnerability with each new generation of people who have not been exposed to either wild-type or vaccine virus, makes the decision to stop vaccinating a particularly awesome one."

In another article in the journal, Dr Paul Fine, of the Department of Infectious and Tropical Diseases at the London School of Hygiene and Tropical Medicine, writes: "The cessation of oral poliovirus vaccine will introduce an epidemiological situation which has never before been experienced, and will require intense and imaginative implementation and monitoring to ensure its success."

After eradication, the only source of wild polioviruses will be laboratories. WHO has issued a global action plan and timetable for the safe handling and maximum containment of these and other potentially infectious materials. Laboratories are being asked to impose rigorous safety procedures and eventually to destroy their stocks or have them transferred to WHO-designated repositories.

Dr Vincent Racaniello, professor of microbiology at Columbia University College of Physicians and Surgeons, New York, warns of the "mind-boggling scale of tracking down all the poliovirus stocks." For example, he writes that some laboratories may not have adequate inventories of all their stocks, and that it will take more than "goodwill" on the part of nations to get a clear picture of the existing laboratories holding poliovirus.

Summarizing the debate over how and when to stop immunization after eradication, authors Wood, Sutter and Dowdle write: "Because of the potential risks and benefits inherent in such a decision, the best available science, a risk-benefit analysis, contingency plans, a stock pile of poliovirus vaccines, and the endorsement by the global policy-making committees will all be needed before vaccination can be discontinued."

For further information please contact Gregory Hartl, WHO Press Spokesperson, Geneva. Tel (+41 22) 791 4458, Fax (+41 22) 791 4858. E-mail: hartlg@who.int

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