Spillover rate is not predictive of host jump risk

Host jumps--defined as the process by which a pathogen establishes sustained transmission in novel hosts--are threats to human and animal welfare, but prediction of successful host jumps remains elusive. A spillover event must precede a host jump, and so spillover rate is thought to be related to risk. However, non-endemic pathogens that spill over frequently have demonstrated a poor ability to host jump from any given spillover. So which is riskier, pathogens that spill over rarely or commonly? Applying a Bayesian framework to a general model of host jump risk, we show that 1) the riskiest pathogens can be those that spill over at low, intermediate, or high rates, and 2) as the rate of spillover gets large, the information gained from past spillovers is exactly counterbalanced by the increased number of future spillovers. Taken together, this means that spillover rate has little to no value for explaining host jump risk. Rather, we show that novel pathogens (i.e. pathogens with a relatively short history of spilling over in their current form) are substantially more likely to result in host jumps than pathogens that have had long-associated opportunities for spillover into the novel host. Therefore novelty, but not spillover rate, is an important predictor of host jump risk. Significance StatementSpillovers of infectious diseases pose significant threats to human and animal health in part due to pandemic risk and the potential for successful host jumps. Predicting successful host jumps remains elusive, though it is often assumed that increased spillover rates are associated with increased host jump risk. We developed a quantitative framework that uses past spillover outcomes to predict future host jump risk. Our model demonstrates that spillover rate is not predictive of host jump risk. However, pathogens that recently started having opportunities for spillover are substantially riskier than those that have a longer history of past spillover. We therefore show that pathogen novelty is a much stronger driver of host jump risk than spillover rate. This ideological shift may significantly enhance our ability to identify contexts where host jumps are most likely, yielding substantial economic and public health benefits globally.