Short and long term suppression of host populations by novel pathogens

Current theory for the regulation of host populations by pathogens suggests that a high level of suppression during the initial epidemic phase will be followed by a population rebound with decreased virulence due to pathogen and host evolution, and the extent of host suppression increases with increasing pathogen transmissibility ( $R_0$ ) and virulence. Using simple epidemiological models, we explore the effect of three factors on short- and long-term suppression: the strength of density-dependent population regulation (homeostasis); maternal antibodies, and age-dependent mortality. Rapid homeostasis can mitigate long-term population suppression, and surprisingly weak homeostasis can even result in a greater population suppression during the endemic phase compared to the initial epidemic. Maternal antibodies can significantly reduce suppression of the host population if they attenuate rather than block infections. A similar result obtains if the severity of disease is lower in the young than in adults. In both cases, a higher $R_0$ can result in lower suppression, and the average virulence can decline over time without any (genetic) evolution. Our results suggest the need for a nuanced view of long-term suppression by a new pathogen, with the outcome sensitive to many details even in the absence of evolution.