Anna C Fagre, Lily Cohen, Evan A. Eskew, Maxwell Jenner Farrell, Emma Glennon, Maxwell B Joseph , Hannah K. Frank, Sadie Jane Ryan, Colin J. Carlson, Gregory Albery

The SARS-CoV-2 pandemic has led to increased concern over transmission of pathogens from humans to animals (“spillback”) and its potential to threaten conservation and public health. To assess this threat, we reviewed published evidence of spillback events, including instances where spillback could threaten conservation and human health. We identified 97 verified examples of spillback, involving a wide range of pathogens; however, infected hosts were mostly non-human primates or large, long-lived captive animals. Relatively few spillback events resulted in morbidity and mortality, and very few led to maintenance of a human pathogen in a new reservoir or subsequent “secondary spillover” back into humans. Together, these results imply that spillback represents an apparently minor threat to conservation and public health, particularly relative to other anthropogenic stressors like land use and climate change. Lastly, we outline how researchers can collect experimental and observational evidence that will expand our capacity for spillback risk assessment.