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Open Access Highly Accessed Research article

Spatial-temporal excess mortality patterns of the 1918–1919 influenza pandemic in Spain

Gerardo Chowell12*, Anton Erkoreka3, Cécile Viboud1 and Beatriz Echeverri-Dávila4

Author Affiliations

1 Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, MD, USA

2 Mathematical, Computational & Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA

3 Basque Museum of the History of Medicine and Science. University of the Basque Country, Bilbao, Spain

4 Grupo de Estudios de Población y Sociedad, Universidad Complutense de Madrid, Madrid, Spain

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BMC Infectious Diseases 2014, 14:371  doi:10.1186/1471-2334-14-371

Published: 5 July 2014

Abstract

Background

The impact of socio-demographic factors and baseline health on the mortality burden of seasonal and pandemic influenza remains debated. Here we analyzed the spatial-temporal mortality patterns of the 1918 influenza pandemic in Spain, one of the countries of Europe that experienced the highest mortality burden.

Methods

We analyzed monthly death rates from respiratory diseases and all-causes across 49 provinces of Spain, including the Canary and Balearic Islands, during the period January-1915 to June-1919. We estimated the influenza-related excess death rates and risk of death relative to baseline mortality by pandemic wave and province. We then explored the association between pandemic excess mortality rates and health and socio-demographic factors, which included population size and age structure, population density, infant mortality rates, baseline death rates, and urbanization.

Results

Our analysis revealed high geographic heterogeneity in pandemic mortality impact. We identified 3 pandemic waves of varying timing and intensity covering the period from Jan-1918 to Jun-1919, with the highest pandemic-related excess mortality rates occurring during the months of October-November 1918 across all Spanish provinces. Cumulative excess mortality rates followed a south–north gradient after controlling for demographic factors, with the North experiencing highest excess mortality rates. A model that included latitude, population density, and the proportion of children living in provinces explained about 40% of the geographic variability in cumulative excess death rates during 1918–19, but different factors explained mortality variation in each wave.

Conclusions

A substantial fraction of the variability in excess mortality rates across Spanish provinces remained unexplained, which suggests that other unidentified factors such as comorbidities, climate and background immunity may have affected the 1918–19 pandemic mortality rates. Further archeo-epidemiological research should concentrate on identifying settings with combined availability of local historical mortality records and information on the prevalence of underlying risk factors, or patient-level clinical data, to further clarify the drivers of 1918 pandemic influenza mortality.

Keywords:
1918–1919 influenza pandemic; Spain; Spanish influenza; Spring-summer wave; Excess death rates; Relative risk of death; Transmissibility; Provinces; Geography; Spatial heterogeneity