Population Density Doesn't Solve the Puzzle of COVID-19 Spread
When looking at comparisons of COVID-19 mortality rates between countries, or between US states, many casual observers in social media and publications' comment sections are often quick to point to population density as the overwhelming and deciding factor in determining overall infection.
This is often simply assumed to be self-evident. But when we look at differences in COVID-19 deaths—say, differences between Sweden and the UK—this is not really as obvious as many people seem to assume.
In many cases, the "evidence" provided relies far too much on aggregation, and in other cases, the assumption that greater density leads to greater deaths ignores the fact that denser areas often bring with them mitigating factors—such as greater access to healthcare institutions—that may lead to lower mortality rates in them, even if infection rates are higher.
The Uselessness of Average Density Rates
The most lazy approach to making pronouncements on the effects of density often relies on simply calculating population densities for entire countries. So, if we're comparing Sweden with the UK, we merely have to look at density—720 pop./sq. mi. in the UK, and 59 pop./sq. mi. in Sweden—to conclude "Voilà! This is why the UK has a much higher number of COVID-19 deaths per capita."
More astute readers will see the problem here immediately. This assumption only works if population is more or less distributed evenly across a jurisdiction. In practice, however, many countries are characterized by a few small dense areas surrounded by much larger areas with very low population density.
And how big are these urban areas? They constitute 8 percent of the land in the UK and 1.3 percent of the land in Sweden.
We find similar issues in the United States. Many of the states that are often assumed to be "rural" in nature are often nothing of the sort. In Colorado, for instance, 86 percent of the population lives in "urban areas." In other words, very little of the population is rural. Most of the state is unoccupied land.
But even when we do this, we're left with too much aggregation. Not all urban areas are equally dense. Large variations remain here as well. Some urban areas could have sizable districts with single-family houses. Other areas could be large apartment blocs piled on top of each other. Making useful comparisons would require a lot more work than simply looking at a country's average population density.
Does Research Show More Death from Higher Density?
Fortunately, some researchers have attempted to drill down deeper in order to examine the relationship between infectious diseases and population density.
But even when these variations are accounted for we're still not left with conclusive evidence that population density produces more mortality from infectious diseases.
In a June 2020 study in the Journal of the American Planning Association on COVID-19 fatality, the authors conclude:
counties with higher densities have significantly lower virus-related mortality rates than do counties with lower densities….we find no evidence that sprawling areas are more immune to the pandemic or that sprawling areas experience lower death rates. Indeed, we find that pandemics are deadlier in low-density areas that have less access to quality health care.
This is counterintuitive in many ways, of course. It certainly stands to reason that that more contacts among more people would lead to infection. But more infection doesn't necessarily lead to more death. After all, low-density areas are often relatively poor, or at least no better off economically than the urban core in terms of income. In terms of amenities like hospitals, however, urban core areas have more access than rural areas of similar income levels.
Similarly, Richard Florida, writing in Bloomberg's CityLab found that while some dense areas like New York City were indeed grimly impacted by high rates of mortality, some far less dense areas, such as Albany, Georgia were heavily affected in April were as well. Florida continues:
As to the question of density itself: [Jed] Kolko’s analysis finds density to be significantly associated with Covid-19 deaths across U.S. counties. But density is not the only factor at play. His analysis also finds that Covid-19 death rates per capita are higher in counties with older populations and larger shares of minorities, and colder, wetter climates. It’s important to remember that this analysis only looks at the U.S., and in other parts of the world, denser cities have had more success controlling the spread.
But even in some cases where modern healthcare was not available, there is conflicting evidence on how population density impacts mortality.
This is why Ruiqi Li et al. write in Physica A: Statistical Mechanics and Its Applications that "Investigations of possible links between population density and the propagation and magnitude of epidemics have so far proved inconclusive." The authors refer, for example, to a study on the 1918 influenza epidemic by Gerardo Chowell et al. which states, "we did not find any obvious association between death rates and measures of population density or residential crowding."
This isn't to say that density has no effect on the spread of disease, of course. This does indeed appear to be almost self-evident. The question is whether or not density is the primary factor, or a factor that's more important than other key factors, such as, say, per capita income or other sociodemographic indicators.