The epidemic consequences of virulence and free-living survival relationships in models of emerging outbreaks

The relationship between parasite virulence and transmission is a pillar of evolutionary theory that has specific implications for public health. Part of this canon involves the idea that virulence and free-living survival (a key component of transmission) may have different relationships in different host-parasite systems. Most examinations of the evolution of virulence-transmission relationships (theoretical or empirical in nature) tend to focus on the evolution of virulence, with transmission a secondary consideration. In virus evolution, recent studies have examined how the evolution of free-living survival can influence other traits. However, few studies have examined the epidemic-scale consequences of evolution across the different virulence-survival relationships, and specifically, of increased free-living survival. In this study, we utilize a mathematical model motivated by aspects of SARS-CoV-2 natural history to investigate how evolutionary changes in survival traits, and their subsequent relationship to virulence traits, may influence several aspects of disease dynamics. Specifically, we examine how each of two different hypothesized relationships between virulence and survival, one deemed the Curse of the Pharaoh hypothesis and the other more classically referred to as the tradeoff hypothesis. Across virulence-survival frameworks, we found that small changes (5% above and below the nominal value) in survival can have a meaningful effect on certain outbreak features, including the R0, and the size of the infectious peak in the population. These results highlight the importance of properly understanding the relationship between virulence and parasite survival, as they offer considerably different predictions for how the evolution of free-living survival will influence the trajectory of an emerging epidemic.