It is hoped, and widely expected, that early in the next century the wild-type polio virus will be declared extinct as a result of an eradication programme started in 1988 by the World Health Assembly using live attenuated trivalent oral (Sabin) polio vaccine (OPV).1 This potential achievement is reminiscent of the successful eradication of smallpox in 1977, but there are important differences. Most polio infections are subclinical and those that cause myelitis are not reliably distinguished from other causes of acute flaccid paralysis on clinical grounds alone. Highly effective clinical and laboratory surveillance will be needed to ensure that polio has really gone.2 Most importantly in practical terms, worldwide immunisation will be necessary until global eradication is certified to prevent re-importation and outbreaks in children who have not been immunised. This is confirmed by recent outbreaks in areas where immunisation rates with OPV have fallen, or where pockets of unimmunised subjects have developed—for example, in Albania3 and Chechnya.4 In contrast, during the final years before smallpox eradication, a number of developed countries stopped routine immunisation as it was causing significant morbidity and mortality in the absence of any cases of smallpox, so that the risk:benefit ratio was against continuing. Unlike polio, the importation and spread of the disease was judged to be unlikely to occur and easy to control.

A similar risk:benefit problem now faces countries that have achieved effective control of polio using OPV. Almost all of the small residual number of cases of paralytic polio in these countries are caused by the vaccine virus,5 6 either in recipients of the vaccine (usually children) or in non-immune subjects in contact with recipients to whom the virus is spread (usually adults). Vaccine associated paralytic polio (VAPP) occurs as a consequence of the reversal of attenuating …