The scientific basis for stopping polio immunisation--issues and challenges.

Eradication of polio is in sight. Once this is finally achieved there is general agreement that the world should move from the current policy of universal childhood immunisation, mostly with the oral poliovirus vaccine (OPV), to no immunisation. The challenge ahead is to build consensus on how and when this is achieved. The major area of scientific uncertainty is whether OPV-derived strains excreted by vaccinees will establish chains of transmission in communities, especially in settings of poor hygiene, when population immunity levels begin to fall after immunisation ceases. Evidence from several sources suggests that this may not occur. Most convincing are studies that show polioviruses are not detected beyond three months after mass OPV campaigns in at least two countries that only use OPV in this way. However it is clear that there are significant gaps in our knowledge. These include the transmissibility of OPV-derived strains under certain ecological settings, the potential for persistently infected individuals to re-seed viruses into a community, and the molecular basis of transmissibility. Research in these areas is underway and will help develop and refine strategies for stopping polio immunisation. The potential risks and benefits of ceasing polio immunisation are such that the best available science is required to reach an informed decision.