Speaker
Description
Host immunity drives the evolution of many pathogens towards antigenic escape. However, the contribution towards this escape may not be uniform across the population: different types of hosts may contribute more or less in terms of both immune pressure and onward transmission. Here we investigate the population-level consequences of this heterogeneity, focussing on vaccine escape.
This heterogeneity depends on the immune status of each individual host, especially through (i) vaccination; (ii) previous infection history and (iii) general immunocompetence. Existing host immunity may increase or decrease the contribution to escape, as in other phylodynamic phenomena.
We define the escape pressure as a linear combination of infections in different groups over an epidemic. We find the escape pressure using SIR-style models with imperfect vaccines and variable vaccination coverage. We also consider the stochastic invasion dynamics of escape strains, and their antigenic evolution over multiple epidemic waves.
The effects of vaccination on the escape pressure depend on the relative role of vaccinated hosts. If they do not contribute much more than unvaccinated hosts to the escape pressure, increasing vaccination will always reduce the overall escape pressure. However, if vaccinated hosts contribute to escape significantly more, the escape pressure will peak at intermediate vaccination levels.
However, these patterns are more complicated with more heterogenous population immunity. The different individual contributions to antigenic escape will shape pathogen evolution. At the population-level, these heterogeneous escape contributions can lead to unintuitive effects in the escape pressure, particularly on its dependence on vaccine coverage. Therefore, it is important to understand better these individual escape contributions and their dependence on host immunity.
The considerations here have implications for the design of vaccination strategies, and may help to mitigate the risk of vaccine immune escape in a population.
