Dating the emergence of pandemic influenza

Prospectively collected demographic, hospitalization, and death data from all individuals who served in the Australian Imperial Force from 1914 through 1919 in Europe and the Middle East were abstracted from archived records.

Analyses were conducted to determine mortality risk factors.

Many military organizations maintain detailed administrative and medical information on all of their members for long periods.

Details regarding the origin of the pandemic strain of the virus, its evolution within the human population, and the pathophysiological mechanisms that produced the unprecedented mortality among young adults are of interest to influenza pandemic response planners.

Because the 1918–1919 pandemic predated the advent of virology, insights regarding the pandemic influenza A/ H1N1 virus have been largely obtained from laboratory studies of archived autopsy and permafrost-preserved samples [9–11].

The comparative molecular biology of the few available specimens indicates that the pandemic virus evolved its surface hemagglutinin binding characteristics during 1918; evolution of the hemagglutinin of the pandemic strain likely enabled the epidemic waves [12–14].

However, the origin of the pandemic strain and the timing and location of its emergence in humans remain unclear; prominent hypotheses implicate the central plains of the United States (in March 1918) [15, 16] and large military camps in France (in the winter of 1916–1917) [17, 18].

The mechanisms of its extraordinary lethality include the inherent pathogenicity of [3, 4] and dysregulated immune responses to the pandemic influenza A/H1N1 virus strain and the effects of secondary bacterial coinfections [5, 6].

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There was a strong inverse relationship between length of service in the Australian Imperial Force and mortality risk from pneumonia-influenza during the fall-winter of 1918–1919.The protective effect of increased service likely reflected increased acquired immunity to influenza viruses and endemic bacterial strains that caused secondary pneumonia and most of the deaths during the 1918–1919 influenza pandemic.

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