An extended period of elevated influenza mortality risk follows the main waves of influenza pandemics

Abstract

Understanding the extent and evolution of pandemic-induced mortality risk is critical given its wide-ranging impacts on population health and socioeconomic outcomes. We examine empirically the persistence and scale of influenza mortality risk following the main waves of influenza pandemics, a quantitative analysis of which is required to understand the true scale of pandemic-induced risk. We provide evidence from municipal public health records that multiple recurrent outbreaks followed the main waves of the 1918-19 pandemic in eight large cities in the UK, a pattern we confirm using data for the same period in the US and data for multiple influenza pandemics during the period 1838-2000 in England and Wales. To estimate the persistence and scale of latent post-pandemic influenza mortality risk, we model the stochastic process of mortality rates as a sequence of bounded Pareto distributions whose tail indexes evolves over time. Consistently across pandemics and locations, we find that influenza mortality risk remains elevated for around two decades after the main pandemic waves before more rapid convergence to background influenza mortality, amplifying the impact of pandemics. Despite the commonality in duration, there is heterogeneity in the persistence and scale of risk across the cities, suggesting effects of both immunity and socioeconomic conditions.

Keywords: Influenza pandemics; Mortality risk dynamics; Post-pandemic period.

Fig. 1

Annual mortality rate from influenza in deaths per million population. The upper y-axis tick indicates the maximum mortality rate for each data series. See Section 2.1 for data sources.

Fig. 2

Annual mortality rate from influenza in England and Wales in deaths per million population. Alternating grey shading delimits the periods associated with each pandemic (e.g. the first light grey area corresponds to the 1847-48 pandemic and subsequent post-pandemic period; see Table 2 for pandemic dates). Because the exceptionally high 1918 mortality sets the scale of the figure, insets show the periods until 1914 and from 1957 scaled to mortality relevant for these periods. See Section 2.1 for data sources.

Fig. 3. Time evolution of outbreak risk following the 1918-19 pandemic.

(a)–(b) Outbreak risk computed from the model fitted to each city, and for thresholds of (a) 500 deaths per million population and (b) 750 deaths per million population. (c)–(d) Outbreak risk computed from the model fitted to data for England and Wales and the United States, for two thresholds.

Fig. 4

Time evolution of outbreak risk following the 1847-48 and 1890-91, 1918-19 and 1968-70 pandemics in England and Wales, for different thresholds corresponding to proportions of the maximum mortality observed during the main waves of the corresponding pandemic.

Fig. 5

Scatterplots showing (a) the maximum outbreak risk plotted against main wave mortality and (b) the average slope of the outbreak risk over the first post-pandemic decade. In both panels, main wave mortality is measured as the average mortality per million population over the main waves of the 1918-19 pandemic, and the outbreak risk mortality threshold is 500 per million.

Fig. 6

Scatterplots showing the relationship between average influenza mortality during the first post-pandemic decade and (a) the average influenza mortality rate per million population during the main waves and (b) pre-pandemic all-cause infant mortality as deaths per 1000 births (averaged over 1895-1917).

Fig. 7

Scatterplots showing (a) the maximum outbreak risk and (b) the average slope of the outbreak risk over the first post-pandemic decade, plotted against pre-pandemic all-cause infant mortality as deaths per 1000 births (averaged over 1895–1917).