Is Urban Density a Key Factor in the Spread of Covid-19?

A new peer-reviewed paper examines the literature on the influence of urban density on the diffusion of Covid-19.

Public discussion of Covid-19 has often tried to make a connection between urban density and the rate of virus diffusion. The primary assertion is that more densely populated areas have experienced a faster spread of Covid-19. In Jacques Teller's recent Buildings & Cities synthesis paper, URBAN DENSITY AND COVID-19: TOWARDS AN ADAPTIVE APPROACH, he compares research studies to consider whether this is actually the case. There is no scientific consensus about urban density’s role in the spread of Covid-19. Instead, urban connectivity (the movement of people between cities) is likely to play a bigger role in the diffusion of the pandemic.

A total of 15 recent papers on the connection between urban density and Covid-19 are examined. Across these statistical studies, Teller finds the connection between urban density and the diffusion of Covid-19 is a complex and disputed issue. Teller highlights that all of the studies acknowledged the multifactorial nature of Covid-19 diffusion, and often incorporated several factors into their analysis. Six core factors were identified in the different papers:

• Urban settlement: This is comprised of urban density (number of inhabitants/km²), internal mobility (between urban, peri-urban and rural areas) and external connectivity (flows of people between cities)
• Socioeconomic characteristics:  Mean age; life expectancy; personal economic circumstances; household composition; race and ethnicity.
• Urban services: For example, the presence of healthcare and hospitals.
• Urban environment: Air pollution; the concentration of particulate matter; air temperature and relative humidity.
• Non-pharmaceutical interventions adopted: Social distancing; face-covering and hand-sanitising; travel restrictions; stay-at-home measures; obligations to quarantine.
• Time: e.g. the delay between the start of the epidemic and the adoption of containment measures.

Of the six factors there was no single factor that clearly emerged as having a larger influence than others. Instead, the factors are closely intertwined in shaping the diffusion of the epidemic. In addition, each study only considered a sub-set of the factors listed. This divergence, along with several other differences made it difficult to compare and draw conclusions from the studies.

Crucially, the papers do not have a shared definition of urban density, nor do they all use the same dependent variable (this includes: total number of reported cases; number of reported cases/100,000 inhabitants; and number of reported deaths/ 100,000 inhabitants). Teller suggests that approaches to testing and tracing need to be incorporated into analyses, since differences in how cases and deaths were reported and counted will influence outcomes.

An important distinction between urban density and connectivity was not made across the papers. In studies that did distinguish between density and connectivity, the influence of urban density on the diffusion of Covid-19 tends to be reduced or negative. Only a small number of the studies considered external connectivity, even though it is widely acknowledged to play a crucial role in the diffusion of diseases in a globalised world. Teller notes that denser and larger cities are usually more connected; ignoring external connectivity may therefore lead to overestimations and an overemphasis of the role of urban density.

A number of gaps provide a valuable starting point for future enquiry. Teller suggests that:

• A more sophisticated approach to defining and analysing density is necessary: mean density is too aggregated ‘to reveal the strength of connectivity and urban flows in the city’
• There is a need to address the unbounded and topological nature of the city
• Overcrowding and density may be better addressed at the building level, and related to residential or work activities
• Analysts must pay increased attention to construction techniques and building operation systems (see Edward Ng’s commentary From SARS to COVID-19 and Beyond: Public Health Lessons for Buildings for more on this)
• Analysis needs to address divergence in governance styles and structures
• Studies are urgently needed to explore the role of urban density in economically developing countries, particularly in informal settlements.

It is only through synthesis of the type that Teller has undertaken here that we can see the patterns and gaps emerging in approaches to study this complex phenomenon. Through this, more conclusive analysis and clear ways forward for managing virus spread in urban environments can be developed.

Teller J. (2021). Urban density and Covid-19: towards an adaptive approach. Buildings and Cities, 2(1), pp. 150–165. DOI: https://doi.org/10.5334/bc.89

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