High population densities catalyse the spread of COVID-19.

TLDR: In addition to isolation of ill persons, contact tracing and quarantining of all their contacts, to reduce community spread it will be necessary to strategically reduce contact-rates.

Abstract: The control of the COVID-19 epidemic is in many locations moving from a public-health strategy of containment to mitigation. 1 A main control-strategy of COVID-19 is contact tracing. Its effectiveness depends on the pre-symptomatic and asymptomatic patterns of the disease. With 100% symptomatic cases, an R0 of 1.5 could be controlled with 50% of the contacts traced. With an R0 of 3.5, 90% is required. 2 With pre-symptomatic and potential asymptomatic transmission, the effectiveness of contact tracing is reduced further. 2 In Italy, for example, only one out of four cases is identified. 3 Thus, even for a low R0 and no presymptomatic transmission, contact tracing will on its own not be able to contain the outbreak. In addition to isolation of ill persons, contact tracing and quarantining of all their contacts, to reduce community spread it will be necessary to strategically reduce contact-rates. By reducing contact rates, the growth-rate of the outbreak can be reduced. Controlling contact rates is key to outbreak control, and such a strategy depends on population densities.