Future Challenges

Eradicate Polio: Then What?

Ouite likely, poliomyelitis will be among the next ancient scourges to follow smallpox down the road to eradication. However, with smallpox, for a variety of political and scientific reasons, the causative virus still exists many years after the last naturally acquired case of the disease. Will it be any easier to rid the world of the polioviruses after the last case of poliomyelitis occurs?

Dramatic progress toward polio eradication has mainly been accomplished using "immunization days" when the entire population in a given area is immunized using attenuated (Sabin) vaccine, often with one or more follow-up days to immunize individuals missed earlier. The vaccine viruses then disappear from the population within a few months. Although a small percentage of the population suffers paralytic illness from the vaccine, the pop ulation is protected from virulent polioviruses until new generations are born. If there were no polioviruses left in the world, there would be no need to immunize the new generations.

Potential sources of virulent polioviruses include wild viruses circulating in remote populations, individuals with mild or atypical illness, long-term carriers (especially those with immunodeficiency, who excrete the viruses for months or years), and laboratory freezers around the world that are known to harbor the viruses, as well as frozen specimens such as feces that could harbor them. It is even possible to synthesize a poliovirus in the laboratory. Lastly, vaccine viruses can change genetically and acquire full virulence as occurred in the Hispaniola polio epidemic of2000-2001.

The challenge is to have a continuous reliable global polio surveillance system, and maintain immunizations and strategically located stockpiles of vaccine during what is likely to be a long time after the last case of paralytic polio occurs.

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