ERADICATION OF POLIOMYELITIS - WHEN CAN ONE BE SURE THAT POLIO VIRUS TRANSMISSION HAS BEEN TERMINATED

Most polio virus infections are silent. Vaccination reduces the incide nce of infection, and the period between clinical cases of poliomyelit is becomes longer. As the point of eradication is approached, it becom es increasingly difficult to use the case-free period to determine whe ther silent infections have ceased. In this paper, the authors use sto chastic computer simulations to relate case-free periods to the presen ce or absence of silent infections. After 2 years without paralytic ca ses in a population of 200,000 inhabitants, the probability for the pr esence of silent infections can still be as high as 38%. The case-free period must exceed 3 years before one can be 95% certain that there h as been local extinction of the wild polio virus infection. Even after 5 years without cases, the probability of silent polio virus transmis sion can still be in the range of 0.1-1.0%.