Mortality burden of the 1918-1919 influenza pandemic in Europe

Abstract

Background: The origin and estimated death toll of the 1918-1919 epidemic are still debated. Europe, one of the candidate sites for pandemic emergence, has detailed pandemic mortality information.

Objective: To determine the mortality impact of the 1918 pandemic in 14 European countries, accounting for approximately three-quarters of the European population (250 million in 1918).

Methods: We analyzed monthly all-cause civilian mortality rates in the 14 countries, accounting for approximately three-quarters of the European population (250 million in 1918). A periodic regression model was applied to estimate excess mortality from 1906 to 1922. Using the 1906-1917 data as a training set, the method provided a non-epidemic baseline for 1918-1922. Excess mortality was the mortality observed above this baseline. It represents the upper bound of the mortality attributable to the flu pandemic.

Results: Our analysis suggests that 2.64 million excess deaths occurred in Europe during the period when Spanish flu was circulating. The method provided space variation of the excess mortality: the highest and lowest cumulative excess/predicted mortality ratios were observed in Italy (+172%) and Finland (+33%). Excess-death curves showed high synchrony in 1918-1919 with peak mortality occurring in all countries during a 2-month window (Oct-Nov 1918).

Conclusions: During the Spanish flu, the excess mortality was 1.1% of the European population. Our study highlights the synchrony of the mortality waves in the different countries, which pleads against a European origin of the pandemic, as was sometimes hypothesized.

Figure 1

Methods: calculation of excess mortality (France). The cross‐hatched area represents excess mortality, calculated as the difference between observed (black line) and predicted (continuous gray line) mortality, from the beginning of the pandemic period (light gray shading; first month during which observed deaths exceeded the pandemic threshold, dashed gray line) until the end (first month during which observed deaths fell below the pandemic threshold).

Figure 2

Excess‐mortality rates* cumulated throughout the entire excess‐mortality period in each European country. Note the north–south gradient in the progressively darker shades of gray, *(/10 000 inhabitants).

Figure 3

Monthly (or trimesterly) mortality rate* (ordinate) in 14 European countries from 1917 to 1921 (abscise). Lines and shading are as defined in the legend to Figure 1, *(/10 000 inhabitants).

Figure 4

Geographic spread of the excess‐mortality peaks.

Figure 5

Comparison of published mortality estimates in Europe (number of deaths) during the 1918–1919 influenza pandemic. Circle: Johnson and Mueller (J); Dark circle: Ansart et al. (A); square: Patterson and Pyle (P); triangle: Murray et al. (M).

Figure 6

P&I‐deaths plotted versus all‐cause excess mortality in 11 European countries*. Spearman’s rank correlation ρ = 0·94; P < 10⁻⁴. *Germany, Spain, France, Italy, Norway, Netherlands, Portugal, Sweden, Switzerland, England and Wales and Scotland.