The smallpox and poliomyelitis (polio) viruses were, at a time, one of the largest threats to global public health killing millions until global eradication campaigns were put into effect. Vaccination led to the eradication of smallpox and the elimination of polio for most of the world. However, polio continues to persist at endemic levels in Pakistan and Afghanistan. We developed ODE models of smallpox and polio to explore differences in transmission dynamics and determine if the underlying biology has made poliomyelitis more difficult to eradicate. Our model analysis shows there are multiple factors which should allow polio to have a lower threshold for eradication than smallpox: a lower threshold for herd immunity, and vaccines that are more effective at reducing infections and deaths. Thus, our model analysis leads us to conclude that the persistence of polio is due to the persistence of inadequate vaccination rates in the remaining polio-endemic countries.



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