Cross protection between the first and second waves of the 1918 influenza pandemic among soldiers of the Canadian Expeditionary Force (CEF) in Ontario.

OBJECTIVES: This research analyses morbidity and mortality during the 1918 influenza pandemic among Ontario soldiers in the Canadian Expeditionary Force (CEF). This paper asks: did exposure to influenza during the first wave confer protection against illness and death during the second wave of the pandemic? METHODS: Pneumonia and influenza (P&I) cases and deaths among Ontario soldiers were transcribed from the 1918 Admission and Discharge books for the CEF. Following the methods of Barry et al. [10], hospital admission and mortality rates for P&I were compared for new recruits (<1 month service) and seasoned soldiers (>1 month service) in order to assess the possibility of cross protection during successive waves of the pandemic. RESULTS: The first wave of the 1918 influenza pandemic occurred between March and May of 1918, with the second wave erupting from September to December. Mortality in the second wave was more severe, with a case fatality rate of 4.7%, which was more than double the rate of 2.3% from March to May. Seasoned soldiers experienced 82.5% protection from illness due to P&I illness in the fall, and 84% protection from death. CONCLUSIONS: The morbidity data for the soldier population of Ontario, data unavailable for civilians, confirms the presence of a herald wave in Ontario. The findings support the hypothesis that exposure to influenza during the first wave of the pandemic had a protective effect during the second more deadly wave in the fall. Regional heterogeneity characterized the pandemic among soldiers in Ontario. Conscription practices may have funnelled vulnerable recruits, such as rural farmers, into training camps after the first wave of the pandemic, but prior to the second wave.

The first wave of the 1918 influenza pandemic among soldiers of the Canadian expeditionary force.

OBJECTIVES: This article evaluates the evidence for the presence of the first, mild wave of the 1918 influenza pandemic among soldiers in the Canadian Expeditionary Force (CEF). METHODS: Death records for soldiers in the CEF who died in Canada in 1917 and 1918 were extracted from the Commonwealth War Graves Commission and record-linked to the Canada War Graves Registers, Circumstances of Casualty database. Monthly mortality rates from pneumonia and influenza (P&I) were compared with mortality rates from all other causes for 1917 and 1918, and by region for 1918. RESULTS: The herald wave of influenza was present among CEF soldiers in 1918. P&I mortality was significantly higher in March and April 1918 than during the same period in 1917. P&I mortality rates varied across the country and were significantly higher among soldiers who died in the Maritime region of Canada. In March, Maritime P&I mortality was significantly higher than its counterpart in the West; in April it was significantly higher than P&I mortality in both the Central and Western regions. CONCLUSIONS: The CEF findings suggest that local, geographic heterogeneity characterized the first wave of the 1918 influenza pandemic in Canada and illustrate the ways in which well-established, historical patterns of cross-border social contact with the United States, coupled with the special conditions created by warfare, disproportionately funnelled influenza into particular regions. Identification of the mild first wave among soldiers in the CEF calls for more research on the civilian experience of both waves of influenza in Canada.