Epidemiologic characterization of the 1918 influenza pandemic summer wave in Copenhagen: implications for pandemic control strategies

Abstract

Background: The 1918-1919 A/H1N1 influenza pandemic killed approximately 50 million people worldwide. Historical records suggest that an early pandemic wave struck Europe during the summer of 1918.

Methods: We obtained surveillance data that were compiled weekly, during 1910-1919, in Copenhagen, Denmark; the records included medically treated influenza-like illnesses (ILIs), hospitalizations, and deaths by age. We used a Serfling seasonal regression model to quantify excess morbidity and mortality, and we estimated the reproductive number (R) for the summer, fall, and winter pandemic waves.

Results: A large epidemic occurred in Copenhagen during the summer of 1918; the age distribution of deaths was characteristic of the 1918-1919 A/H1N1 pandemic overall. That summer wave accounted for 29%-34% of all excess ILIs and hospitalizations during 1918, whereas the case-fatality rate (0.3%) was many-fold lower than that of the fall wave (2.3%). Similar patterns were observed in 3 other Scandinavian cities. R was substantially higher in summer (2.0-5.4) than in fall (1.2-1.6) in all cities.

Conclusions: The Copenhagen summer wave may have been caused by a precursor A/H1N1 pandemic virus that transmitted efficiently but lacked extreme virulence. The R measured in the summer wave is likely a better approximation of transmissibility in a fully susceptible population and is substantially higher than that found in previous US studies. The summer wave may have provided partial protection against the lethal fall wave.

Fig. 1

First 3 waves of the 1918–1919 pandemic in Copenhagen—monthly incidence of medically treated influenza illnesses (A), influenza hospitalizations (B), and respiratory deaths (C), during 1910–1919 (red lines). Serfling seasonal regression model baselines are shown as blue cyclical lines. The summer pandemic wave, peaking during July 1918, is characterized by substantial morbidity and low mortality.

Figure 2

Scandinavian 1918 summer wave—weekly incidence of cases of influenza illness and respiratory deaths in 4 Scandinavian cities, during 1918–1919. Mortality data from Stockholm, Oslo, and Gotenborg are based on data reported by Low [8] and depict pneumonia and influenza (P&I) mortality; data from Copenhagen are based on all respiratory deaths, including bronchitis (nearly 90% of deaths during the 1918 fall wave in Copenhagen were coded to bronchitis).

Figure 3

Algorithm for estimating transmissibility of the 1918–1919 pandemic (R, the reproductive number). A, Relationship between R and the weekly growth factor (γ), for data representing a long, 4-day, serial interval (used by Mills et al. [1]) and a short, 2.6-day, serial interval (used by Ferguson et al. [4]). B, Incidence of clinically treated influenza illness in 3 Scandinavian cities—Copenhagen (×); Gothenborg (○); Oslo (■)—during the summer and fall of 1918. The lines illustrate the fitting procedure to the exponential growth phase for each wave. For Oslo, the first 2 data points (□) suggest the possible timing of the first case in Oslo (see the Appendix, which is available in the online edition of the Journal of Infectious Diseases).