The Burden and Risk Factors for Death from the 1918-19 Influenza Pandemic amongst the New Zealand Military Forces of World War One

BACKGROUND:The 1918-19 influenza pandemic represents one of the worst human pandemics in recorded history. Given the severity of this pandemic, it could be described as a worstcase scenario for guiding future population-based pandemic planning. As this pandemic occurred during the turbulent final stages of World War One (WW1), many of the bestdocumented aspects from this period are held within military files. AIMS: This thesis uses both historical archives and epidemiological methods to describe the course and impact of the 1918-19 influenza pandemic and identify mortality risk factors amongst the WW1 New Zealand Expeditionary Forces (NZEF) personnel. METHODS: Descriptive datasets and other military/historical records were analysed, along with extensive coding of demographic and military variables, to characterise mortality patterns. Investigation of individual military files provided the samples selected for three separate case control studies performed. RESULTS: This thesis documents the severe impact of the pandemic, which was the main cause of death by disease amongst personnel (estimated n=930); representing 5.1% of military deaths occurring during WW1. Personnel located in the Northern Hemisphere experienced two waves of pandemic mortality (the later is previously undocumented), whilst those in the Southern Hemisphere experienced only one pandemic wave. For those onboard HMNZT 107 Tahiti, the impact of crowding and ineffective isolation measures resulted in one of the worst ship outbreaks of pandemic influenza worldwide during 1918-19. A bimodal pattern of age and mortality risk was found in most NZEF sub-populations; along with higher mortality amongst personnel from a rural background, those who left New Zealand for Europe in 1918, and Māori and Pacific peoples personnel. In the multivariate analyses using adjusted odds ratio (aOR) in logistic regression, the following were found in the final models to be associated with statistically significantly increased pandemic mortality risk depending on location (lower risk in Southern Hemisphere when compared to Northern Hemisphere: aOR: 0.61, 95%CI 0.43-0.86): a larger chest measurement, earlier deployment/embarkment year, and aspects of previous hospital admissions. CONCLUSIONS: This thesis is possibly unique in that it has used both population-level and individuallevel data from the 1918-19 period to characterise pandemic mortality patterns. Personnel experienced a cumulative adverse effect on pandemic mortality risk due to foreign military service, although there was some evidence for increased mortality risk amongst arguably immunological naïve military sub-populations. One of the most notable findings is that of a larger chest measurement as a risk factor (which is a previously undescribed indicator of pandemic mortality risk), potentially consistent with a differential immune system response to the new pandemic strain. This thesis documents the heavy mortality burden of the 1918-19 influenza pandemic amongst the NZEF personnel of WW1 and variation in mortality risk by various host and environmental factors. These findings have implications for future pandemic planning as knowledge of the nature of pandemics is largely based on historical study. Furthermore, the contribution and experience of these personnel should not be forgotten or consigned to history: as shown in this thesis, there is still much that can be learned from these New Zealanders