1. Jonathan Rhodes1, 
2. Frank Dunstan2, 
3. Eamon Laird3, 
4. Sreedhar Subramanian1 and 
5. Rose A Kenny4

http://dx.doi.org/10.1136/bmjnph-2020-000110

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Dear Editors,

We read with interest the review by Dr Kohlmeier in which he reported a correlation between COVID-19 mortality among African-Americans across the USA and northern latitude.1 We previously reported a north–south gradient in global COVID-19 mortality but were conscious that lack of ultraviolet exposure and consequent vitamin D insufficiency was not the only possible explanation.2 We have now investigated the relationships between latitude, age of population, population density and pollution with COVID-19 mortality.

COVID-19 mortality per million by country was downloaded from https://www.worldometers.info/coronavirus/ on 18 May 2020.3We included all 117 countries with population >1 million and ≥150 COVID-19 cases. Data by country for population %≥65 years, population density and air pollution (particles of matter <2.5 um diameter µg/m3) were obtained from public sources.4–6 Latitude was entered for each country’s capital city. The hypothesis was that there was no relationship between mortality and latitude below a threshold and that thereafter mortality increased with latitude. Mortality data were log transformed, and piecewise linear modelling was used to explore the relationship with latitude. This was adjusted for %≥65, and pollution and population density were investigated to see if they further explained variability in mortality.

The analysis supported the hypothesis with a threshold of 28° north and a model of zero slope below the threshold, and a linear model above the threshold was fitted. The age adjustment was highly significant (p<0.0005), with an estimated mortality increase of 13.1% (95% CI 6.9% to 19.8%) for each 1% increase in %≥65. Latitude was also significant (p=0.015) with an estimated 4.9% (95% CI 1.0% to 9.0%) increase in mortality for each 1° further north (table 1, figure 1). Countries with higher pollution included many with younger populations, and pollution was negatively associated with mortality but added no significant explanatory power to a model containing latitude and age. Population density expressed per country was not significantly associated with mortality.