Diffusion and Percolation: How COVID-19 Spread Through Populations

I rely on the key concepts of diffusion and percolation to characterize the sequential but overlapping phases of the spread of infection through entire populations during the first year of the COVID-19 pandemic. Data from Los Angeles County demonstrate an extended initial diffusion phase propelled by radial geographic spread, followed by percolation within hotspots fueled by the presence of multigenerational households. Data from New York City, by contrast, reveal rapid initial diffusion along a unique, extensive subway network. Subsequent percolation within multiple hotspots, similarly powered by a high density of multigenerational households, exerted a positive feedback effect that further enhanced diffusion. Data from Florida counties support the generality of the phenomenon of viral transmission from more mobile, younger individuals to less mobile, older individuals. Data from the South Brooklyn hotspot reveal the limitations of some forms of government regulation in controlling mobility patterns that were critical to the continued percolation of the viral infection. Data from a COVID-19 outbreak at the University of Wisconsin—Madison demonstrate the critical role of a cluster of off-campus bars as an attractor for the continued percolation of infection. The evidence also demonstrates the efficacy of quarantine as a control strategy when the hotspot is contained and well identified.