Minimum requirements for the eradication of poliomyelitis

Martin Eichner , 1993
In: Regional European Meeting of the International Epidemiological Association, Jerusalem, Israel

Institute for Medical Biometry, University of Tübingen, Germany

Abstract

Objectives

In 1988 the World Health Organisation pronounced the plan of the world-wide eradication of poliomyelitis till the year 2000. By means of epidemiological models it is investigated how (and whether at all) this aim can be reached.

Methods

Different vaccination strategies are tested by mathematical and computer-aided modelling, taking into consideration the peculiarities of the two vaccines in use: both are highly effective in preventing paralysis due to a polio virus infection. The oral vaccine (OPV) causes an infection in the vaccinee's digestive tract and can spread to other persons, but it often fails in tropical countries. The inactivated vaccine (IPV) works well both in temperate and tropical countries but does not readily prevent polio virus infections.

Results

In most cases local extinction can be reached within ten years, if at least 55% of all new-borns are (effectively) vaccinated with OPV or if at least 80% are vaccinated with IPV.

Most infections with polio viruses are ``silent''. Only about one in 200 infections leads to paralysis. In a vaccinated population of 200,000 inhabitants the case-free period must exceed three years to guarantee local extinction. The chances are about fifty-fifty after merely one or two years without cases.

Local extinction can be reached with quite low vaccination coverage, which is not sufficient to maintain an infection-free state. In this case reintroductions of wild polio virus infections result in disastrous epidemics. Because only few infectins manifest themselves as paralysis, an infection imported to a village or town can spread unrecognised for some months. Infections already occur in many other villages and towns before the first paralysis takes place.

So-called national vaccination days contributed to eradicate poliomyelitis in South- America: all children under five years are vaccinated once or twice a year with OPV. Following the predictions of a model with unstructured population, an (effective) vaccination coverage of only 25% once a year is enough to ensure extinction of wild virus infection within 10 years.

In more realistic models the total population was subdivided into villages and towns. A great proportion of the children from villages is regarded to be reachable at vaccination days, whereas in towns only a smaller proportion can be reached. In scattered compounds nobody at all is vaccinated. Extinction within ten years is very probable, if at least half of the proportion, regarded to be reachable, is (effectively) vaccinated once a year. If annual vaccination days do not take place in the country as a whole, but are designed individually for each city or village, the results remain the same.

Conclusions

Global eradication of poliomyelitis is possible. With the strategy of vaccinating new-borns it might be very difficult to reach the necessary proportion of infants, especially in inaccessible coumpounds and in the slums of greater tropical cities. The strategy of vaccination days can lead to excellent vaccination coverage. Even models with more realistic assumptions about the accessability of the different population subgroups predict local extinction (and finally global eradication) at reasonable annual vaccination coverage.

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