High population densities catalyse the spread of COVID-19

Abstract

To the Editor-in-chief: 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, R0 of 1.5 could be controlled with 50% of the contacts traced. With 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 pre-symptomatic 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. The basic reproduction number (R0) of SARS-COV-2 is often estimated around 2–4 (several times greater than for influenza A).1 The R0 have many determinants, including control measures, such as social distancing and quarantine. Cruise ships are examples of dense mixing of many persons in a confined space over a relatively long period.4 The density of the group of people on board the COVID-19-infected Diamond Princess, quarantined in Yokohama earlier this year, was estimated around four times higher than that in Wuhan, as was also the R0 before the onset of countermeasures.4 Keeping to more than 1-m distance between people coughing and sneezing, as recommended by the WHO,5 becomes more difficult with higher population densities. Therefore, avoiding situations with higher population densities will be a necessary requirement to limit the spread of COVID-19. None declared.