Without a coordinated state or federal response to COVID-19 across the United States, counties are left to weigh the potentially large yet unseen threat of COVID-19 with the economic and societal costs of enacting strict social distancing measures. The immediate and long-term risk of the virus can be difficult to grasp, given the lack of historical precedent and that many cases go undetected. We calculated the risk that there already is sustained community transmission that has not yet been

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... cted. Given the low testing rates throughout the country, we assume that one in ten cases are tested and reported. If a county has detected only one case of COVID-19, there is a 51% chance that there is already a growing outbreak underway. COVID-19 is likely spreading in 72% of all counties in the US, containing 94% of the national population. Proactive social distancing, even before two cases are confirmed, is prudent.