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

J Infect Dis. 2008 Jan 15;197(2):270-8. doi: 10.1086/524065.


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.

Publication types

  • Historical Article
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Age Distribution
  • Age Factors
  • Aged
  • Communicable Disease Control / history*
  • Communicable Disease Control / methods
  • Denmark / epidemiology
  • Disease Outbreaks / history*
  • Disease Outbreaks / prevention & control
  • Disease Outbreaks / statistics & numerical data
  • Evolution, Molecular
  • History, 20th Century
  • Hospitalization / statistics & numerical data
  • Humans
  • Influenza A Virus, H1N1 Subtype* / genetics
  • Influenza, Human / epidemiology
  • Influenza, Human / history*
  • Influenza, Human / mortality
  • Influenza, Human / transmission
  • Middle Aged
  • Regression Analysis
  • Seasons
  • Survival Analysis