Estimating the risk of re-emergence after stopping polio vaccination.

Live vaccination against polio has effectively prevented outbreaks in most developed countries for more than 40 years, and there remain only a few countries where outbreaks of poliomyelitis by the wild strain still threaten the community. It is expected that worldwide eradication will be eventually achieved through careful surveillance and a well-managed immunization program. The present paper argues, however, that based on a simple stochastic model the risk of outbreak by a vaccine-derived strain after the cessation of vaccination is quite high, even if many years have passed since the last confirmed case.

The Evolution of Costly Resistance in Host-Parasite Systems.

Pairwise invadability analysis is used to examine the evolutionary dynamics of host resistance to microparasitic infection. A continuum of strains of the host differs in susceptibility to infection, with less susceptible strains paying a cost resulting in a lower intrinsic growth rate. With a combination of analytical and graphical pairwise invadability analysis, we show that the evolutionary outcome depends crucially on the shape of the constraint function between resistance and its assumed cost in intrinsic growth rates.