Abstract
An open question in epidemiology is why transmission is often
overdispersed, meaning that most new infections are driven by few
infected individuals. For example, around 10\% of
COVID-19 cases cause 80\% of new COVID-19 cases. This
overdispersion in pathogen transmission is likely driven by intrinsic
biological heterogeneity among hosts, i.e. variability in SARS-CoV-2
viral loads. However, host heterogeneity could also indirectly increase
transmission dispersion by driving pathogen adaptation. Specifically,
transmission variation among hosts could drive pathogen specialization
to highly-infectious hosts. Adaptation to rare, highly-infectious hosts
could amplify transmission dispersion by simultaneously decreasing
transmission from common, less-infectious hosts. This study considers
whether increased transmission dispersion can be, in part, an emergent
property of parasite adaptation to heterogeneous host populations. We
develop a mathematical model using a Price equation framework to address
this question that follows the epidemiological and evolutionary dynamics
of a general host-parasite system. The results predict that parasite
adaptation to heterogeneous host populations drives high transmission
dispersion early in epidemics. Further, parasite adaptation can maintain
increased transmission dispersion at endemic equilibria as long as
virulence differs between hosts in a heterogeneous population. More
broadly, this study provides a framework for predicting how parasite
adaptation determines transmission dispersion for emerging and
re-emerging infectious disease.