The 1918-1919 influenza pandemic in England and Wales: spatial patterns in transmissibility and mortality impact

Abstract

Spatial variations in disease patterns of the 1918-1919 influenza pandemic remain poorly studied. We explored the association between influenza death rates, transmissibility and several geographical and demographic indicators for the autumn and winter waves of the 1918-1919 pandemic in cities, towns and rural areas of England and Wales. Average measures of transmissibility, estimated by the reproduction number, ranged between 1.3 and 1.9, depending on model assumptions and pandemic wave and showed little spatial variation. Death rates varied markedly with urbanization, with 30-40% higher rates in cities and towns compared with rural areas. In addition, death rates varied with population size across rural settings, where low population areas fared worse. By contrast, we found no association between transmissibility, death rates and indicators of population density and residential crowding. Further studies of the geographical mortality patterns associated with the 1918-1919 influenza pandemic may be useful for pandemic planning.

Figure 1

Weekly number of influenza-specific deaths during the 1918–1919 influenza pandemic in England and Wales at two different spatial scales: (a) 305 administrative units and (b) 62 counties (Ministry of Health 1920). The influenza mortality data cover 46 weeks comprising the week ending 29 June 1918 and continuing to the week ending 10 May 1919.

Figure 2

Death rates and population size: the Lorenz curves illustrate the distribution of the total number of influenza deaths as a function of population size for the (a) autumn and (b) winter waves, at the administrative unit level. Red curves, rural areas; blue curves, urban areas; black lines (first diagonal) illustrate a constant distribution of death rates (no heterogeneity in death rates).

Figure 3

Power-law relationship between the total number of influenza deaths and population size, following D˜N^g (D, deaths; N, population size; g, exponent), by pandemic wave and spatial scale. The dashed lines represent the best linear fits to the data in log-log scale. The solid black line with a slope of one is shown as a reference to illustrate constant death rates with population size. (a) 305 units, autumn; (b) 62 counties, autumn; (c) 305 units, winter; (d) 62 counties, winter. At the refined spatial scale of administrative units, the slope is ‘sublinear’ for rural areas (red circles, n=58; g=0.71 (95% CI: 0.60–0.83) in autumn and g=0.77 (0.64–0.90) in winter, significantly below 1.0). By contrast for urban areas, the slope is not different from one (blue circles, n=247; g=0.92 (0.84–1.0) in autumn and g=0.94 (0.85–1.03) in winter). At the county level, the power-law exponent was estimated to be 1.02 (0.96–1.08) and 1.08 (1.01–1.14) for the autumn and winter waves, respectively.

Figure 4

Timing of pandemic onset in the (a) autumn (1918) and (b) winter (1919) wave in the 305 administrative units of England and Wales, as a function of population size. Q1–Q4 represent the quartiles of population size (Q1, lowest; Q4, highest).