Mass gatherings as natural experiments: travel pulses reveal determinants of SARS-CoV-2 epidemic synchrony and predictability in U.S. states and counties

Project Summary Variation in epidemic timing and intensity among U.S. communities has exacerbated shortages of vital pub- lic health resources, from testing and contact tracing to hospital beds and health care workers. Understanding when and why communities are most vulnerable to epidemic disease remains a critical unmet need in this and future epidemics. Our study will leverage a set of five mass gatherings (e.g., the Sturgis Motorcycle Ral- lies in 2020 and 2021) to assess the variable impact of travel pulses across a spectrum of U.S. communities. Our study is organized around two principle aims: Aim 1. To quantify how disease incidence in U.S. counties responds to travel pulses from focal mass gatherings in 2020 and then identify predictors of differing responses. To achieve this, we will use statistical models to quantify counties' differing response to travelers as a function of urbanicity, geographic isolation, age structure, and economic status. Aim 2. To quantify how disease predictability and connec- tivity vary among U.S. counties across 2020 and 2021. We will construct and parameterize mechanistic models of county-level transmission, and then assess model forecasting skill over short-term time horizons. This research will provide much-needed national and state summaries detailing which forces most strongly affect disease connectivity and predictability. Our work will provide detailed evaluations of individual counties, including publicly accessible interactive maps illustrating the relative effects of geography, demography, infection history, and vaccination. These products will help stakeholders identify at-risk communities and tailor control efforts to local conditions, and will highlight regions where increased surveillance is warranted.