Extinction and reintroduction of poliomyelitis
The results presented on this page were obtained, using stochastic models. In conrast to the deterministic models, stochastic models can be used to examine effects in populations of realistic size
- Local extinction
The transmission of an infection within a small population can cease simply because of stochastic effects, even without vaccination. In order to establisch endemic transmission, the population must exceed a critical population size. The probability of extinction further increases with vaccination.
- When can we be sure that extinction has occurred?
Only one in about 100 bis 400 infected individuals develop the dreaded clinical signs of ``infantile paralysis''. It is, therefore, not easy to infer the absence of wild virus infection from the absence of clinical cases. Only after 3 to 5 years without a single case allows us to assume with some certainty that wild virus does no longer circulate.