Population density and basic reproductive number of COVID-19 across United States counties

The basic reproductive number (R ₀ ) is a function of contact rates among individuals, transmission probability, and duration of infectiousness. We sought to determine the association between population density and R ₀ of SARS-CoV-2 across U.S. counties, and whether population density could be used as a proxy for contact rates. We conducted a cross-sectional analysis using linear mixed models with random intercept and fixed slopes to assess the association of population density and R ₀ . We also assessed whether this association was differential across county-level main mode of transportation-to-work percentage. Counties with greater population density have greater rates of transmission of SARS-CoV-2, likely due to increased contact rates in areas with greater density. The effect of population density and R ₀ was not modified by private transportation use. Differential R ₀ by population density can assist in more accurate predictions of the rate of spread of SARS-CoV-2 in areas that do not yet have active cases.