Polio Vaccination: Global Eradication and Changes in Vaccination Schedule

Decker M.D.

Aventis Pasteur, Swiftwater, United States

The global program to eradicate polio has achieved substantial success to date. There were 35,251 cases of polio reported worldwide in 1988, and the actual incidence was probably 10-fold higher. Despite markedly improved surveillance, there were only 2,824 confirmed cases in 2000. The last case of polio in the Western Hemisphere occurred in 1991, and the Americas region of the WHO was declared free of polio in 1994. The Western Pacific region, which includes China, Cambodia, Malaysia, Laos, and Vietnam, recorded its last case of polio on 19 March 1997, and also has been certified as polio-free. It is hoped that the European region will achieve certification soon. Most of the world��s polio now is found in the Indian subcontinent and in Africa, but even there, polio case counts are markedly reduced due to extraordinary efforts.

This worldwide progress has rested on four key activities: achieving and maintaining high routine polio vaccination coverage; instituting sensitive surveillance systems for acute flaccid paralysis; introducing supplemental immunization activities, in particular, National Immunization Days; and conducting other supplemental vaccination activities, such as mopping-up campaigns. In 2000, a record 550 million children under 5 years were immunized during intensified NIDs in 82 countries. These successes offer hope that the worldwide eradication of polio may be achieved soon, and stimulate questions as to how our vaccination strategies should evolve as wild polio disappears.

With the regional eradication of wild poliovirus transmission and the marked reduction in polio worldwide, it has become increasingly difficult to justify exposing healthy children to the small but real risk of OPV-associated paralysis, particularly given the availability of an alternate vaccine that is equally effective and free of the risk of vaccine-associated paralytic polio. Presently, most of North America and Europe uses IPV, either alone or in a sequential schedule. With elimination of wild poliovirus circulation in the Americas, the Western Pacific region, and the European region, it is anticipated that many more countries will begin the transition from OPV to IPV.

Oral polio vaccine (OPV) and enhanced inactivated polio vaccine (IPV) have strengths and weaknesses that complement each other. Where polio has been difficult to control, it has been found that the highest efficacy is achieved by using both in a mixed schedule. OPV alone is the vaccine of choice for countries still experiencing transmission of wild poliovirus. In areas where wild poliovirus no longer circulates, a sequential IPV-OPV schedule, or use of IPV alone, appears increasingly attractive.

Recent experiences in Hispaniola, China, and previously in Egypt have shown the capacity for the OPV virus to reacquire the key characteristics of wild poliovirus: neurovirulence and the ability to sustain transmission in susceptible populations. In addition, immunodeficient persons are capable of prolonged excretion of both wild and reverted, neurovirulent vaccine virus. Ultimately, it seems likely that the final phase of the global eradication program will be the withdrawal of OPV and the use of IPV, typically in combination with one or more other EPI vaccines, prior to the permanent cessation of polio immunization.