Same place, different stories: The mortality burden of the 1918-20 influenza pandemic in Denmark

The 1918-1920 Spanish influenza pandemic is iconic, leading in multiple waves to millions of deaths of mostly otherwise healthy young adults. Among the Nordic countries, Denmark experienced the lowest overall low mortality, however, there was considerable variation between regions within Denmark. Some towns experienced unexplained higher mortality than others, and mortality rates were generally higher in rural than in urban communities.

In this paper, we studied the pandemic’s regional mortality burden in rural and urban Denmark. We investigated the effect of factors such as urbanization, access to medical care, and socioeconomic conditions on excess mortality using mixed linear regression models accounting for the correlation between urban and rural regions of the same area. Information on region specific all-cause mortality, influenza incidence, income, population density, and number of doctors, were obtained from annual reports published by Statistics Denmark. Excess mortality was calculated as the difference with the average the surrounding years.

We find that 0,3% of Denmark’s population died during the four waves of the pandemic. There were substantial regional differences with eastern Zealand being hit much harder than northern Jutland. Urbanization appears to have been an important discriminating factor behind influenza mortality, and the Spanish flu can be seen as an urban disease. On a regional scale, factors such as population density and access to medical care were not associated with increased influenza mortality while influenza incidence and socioeconomic conditions were: influenza-like incidence accounted for 48% of the explained variance while income explained 5%. The observed differences in urbanization can mainly be explained by differences in incidence. While this study could not further explain the cause of these differences on the regional scale, diverging patterns between urban and rural regions could be due to local demographic and socioeconomic variations, which lead to more exposure to the pandemic virus.