What explains the variation in COVID-19 death rates across European countries? One factor that has been mentioned since early on in the pandemic is population density.
As a BBC article from May of 2020 states: “We know that the density of population is important for the spreading of coronavirus.” The article compares the U.K. and Italy, noting that “the U.K. is considerably more densely populated”.
And indeed, it seems plausible that the virus would spread more easily in areas where people are crammed tightly together than in areas they’re spread further apart. For example, aerosols might linger in the elevator in a multi-storey building, whereas they’re unlikely to travel between one semi-detached house and the next.
This raises the question of how to measure population density at the level of whole countries. The simplest measure is the just the number of people divided by the total land area.
However, this measure doesn’t take account of urbanisation. An ostensibly sparse country could have vast swathes of land where nobody lives. So while the average population density would be low, the majority of people might still be crammed tightly together in cities.
An alternative measure, population-weighted density, is provided by the E.U.’s Urban Data Platform. To calculate this measure for a particular country, the country is first divided into ‘parcels’ of 1km2. The population density within each parcel is computed. Then the weighted average is taken, with weights equal to the population of each parcel.
For example, suppose a country comprises ten parcels, nine of which are completely empty and one of which has a population of 100 people. The average density would be only 10/km2, but the weighted density would be 100/km2.
Using average population density, the densest country in Europe is the Netherlands. But using the UDP’s measure of weighted density, the densest country is actually Spain. (And contrary to the aforementioned BBC article, Italy is slightly denser than the U.K.) While this might seem counter-intuitive, it is consistent with other evidence.