Polio vaccine: is it time for a change?

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). This potential achievement is reminiscent of the successful eradication of smallpox in 1977, but there are important diVerences. 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 eVective clinical and laboratory surveillance will be needed to ensure that polio has really gone. 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 Albania and Chechnya. 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 eVective 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, 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 mutations in the vaccine virus—events that occur extremely frequently, 8 but only rarely with adverse consequences in immunologically normal subjects. The overall incidence of VAPP (defined as cases of permanent paralysis) is estimated to be one in 400 000 to one in 520 000 first doses, and one in 12.3 million subsequent doses. 10 A minority of cases of VAPP are caused by humoral or combined immunodeficiency in the recipient. Of 28 confirmed cases of polio in the UK between 1985 and 1994, 19 were VAPP, six were acquired abroad, and three were of unknown origin. As immunisation cannot stop at present, there would be no dilemma about VAPP except that an alternative highly eVective vaccine is now available: enhanced potency trivalent inactivated (Salk) polio vaccine or eIPV. Inactivated vaccine has been used successfully to control polio in several Scandinavian countries and in the Netherlands since the 1950s. This vaccine does not cause VAPP as cases are not reported in these countries. So although the widespread use of OPV is being promoted vigorously in countries in which polio is, or was until recently, endemic, its continued use is being questioned in countries where indigenous polio has disappeared, just as with the smallpox vaccine in the 1970s. Criteria have been drawn up, which countries considering the change should fulfil and include the absence of indigenous poliomyelitis for at least five years and reliable vaccine coverage above 90%. Debate about the relative merits of these two vaccines is not new. It was conducted with vigour by their inventors Salk and Sabin from the 1960s until their deaths, 12 and is continued by many others. 14 Many countries have used the two vaccines at diVerent times and in diVerent ways over the past 40 years. France has moved progressively away from OPV since the early 1980s. More recently, Canada has almost entirely switched to IPV and Germany is now set to do the same. A third alternative to the exclusive use of either OPV or IPV is to adopt a schedule that involves the two vaccines. There is extensive experience of this approach in Denmark and Israel. Current combined schedules usually start with two doses of IPV in infancy followed by OPV boosters in the second year and preschool. The logic for the approach is that the use of IPV should reduce the incidence of VAPP, which most commonly occurs after the first dose of OPV, by inducing suYcient serum neutralising antibody to prevent invasive infection on subsequent exposure to OPV. At the same time the schedule is postulated to retain, to some extent, the perceived benefits for continued use of OPV and seems to be at least as immunogenic as conventional schedules. 18 Austria and Italy have opted for combined schedules. In the USA, where the matter has been under vigorous debate for several years, and where approximately five to 10 cases of VAPP each year have been notified and confirmed in recent years, 19 20 advisory bodies have endorsed a move towards the use of IPV, while leaving open to parents and doctors the options of using either OPV alone or IPV alone. 22 The recommendation for the combined schedule is based on an estimated prediction of a 50% consequential reduction in the incidence of VAPP. Some cases will continue to occur as long as OPV is in use, however, and these moves are likely to precede a complete switch to IPV in the USA in due course. Arch Dis Child 1998;78:571–574 571