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.