I would think that the immunity from a vaccine would last for
quite some time. But for twenty years? I wouldn't want to risk it.
What is of a greater concern is the fact that the polio virus is
non-enveloped. This means it can survive for years outside of a host. So
if they destroyed the stocks a la smallpox, could there be a recurrence of
the disease leaving us vaccine-less? (I don't think vaccine-less is a
word but oh well)