Strengths and weaknesses of current polio vaccines--a view from industry.

Polio eradication is within our grasp and, unless something terribly wrong and unexpected happens, the three types of wild polioviruses will cease to circulate in human populations within the next few years. This achievement will be a result of the rational use of OPV. A momentous global decision--discontinuation of vaccination--will then have to be taken. The most important uncertainty that will weigh upon that decision is whether wild polioviruses can re-emerge after "eradication" defined as "complete interruption of wild polioviruses transmission", has been obtained. It is important to realise that "eradication" does not mean "extinction" in the sense that the dodo is extinct. After eradication, wild polioviruses will still lurk in laboratory specimens and in protected environmental sites (like glaciers) and may even "re-emerge" by back mutation or recombination of Sabin-derived strains that may continue to circulate even after OPV use is discontinued. Theoretically, the risk of re-emergence of wild polioviruses would be lessened if IPV was used for a number of years to immunise all those born after cessation of OPV usage. But the question is "by how much?". Vaccination with IPV will reduce the risk that persistent OPV-derived strains (e.g. in immunodeficient patients) will have the chance to establish permanent transmission after vaccination is totally discontinued. However, the risk of re-emergence will not be changed since this will be determined by the risk of accidental re-introduction. Whether the expense of switching completely from OPV to IPV globally can be justified will depend upon the relative risks of wild poliovirus re-emergence from either OPV-derived sources or other environmental sources including "escape" of virulent seed viruses from IPV production facilities. This balance of probabilities and risks will be very difficult to determine. In any case, it is likely that the decision to upscale IPV production to required levels has already been delayed too long so that polio eradication will be achieved by the use of OPV in developed as well as in less developed countries that cannot afford to use IPV at a high enough vaccine coverage rate to make it safe. Wild poLiovirus transmission has been interrupted with OPV in the Western Hemisphere. There is no reason why this cannot be done in the rest of the world. In industrialized countries that can afford it and where vaccine coverage is sufficient to prevent wild virus circulation, IPV, in combined vaccines, will be increasingly used. Let us hope that politicians in developing countries and zealous ethicists in the developed world will understand why, in the present and foreseeable future circumstances, OPV is better than IPV in the poorer countries and will not demand, in the name of equity in health, a total switch to IPV. For eradication, IPV cannot, and hopefully need not, replace OPV. At this stage it should not.