Risk factors for death from pandemic influenza in 1918–1919: a case–control study

Background
Despite the persisting threat from future influenza pandemics, much is still unknown about the risk factors for death from such events, and especially for the 1918–1919 influenza pandemic.

Methods
A case–control study was performed to explore possible risk factors for death from pandemic influenza among New Zealand military personnel in the Northern Hemisphere in 1918–1919 (n = 218 cases, n = 221 controls). Data were compiled from a Roll-of-Honour dataset, a dataset of nearly all military personnel involved in the war and archived individual records.

Results
In the fully adjusted multivariable model, the following were significantly associated with increased risk of death from pandemic influenza: age (25–29 years), pre-pandemic hospitalisations for a chronic condition (e.g. tuberculosis), relatively early year of military deployment, a relatively short time from enlistment to foreign service, and having a larger chest size (e.g. adjusted odds ratio for 90–99 cm versus <90 cm was 2·45; 95% CI=1·47–4·10). There were no significant associations in the fully adjusted model with military rank, occupational class at enlistment, and rurality at enlistment.

Conclusions
This is one of the first published case–control studies of mortality risk factors for the 1918–1919 influenza pandemic. Some of the findings are consistent with previous research on risk factors (such as chronic conditions and age groups), but others appear more novel (e.g., larger chest size). As all such historical analyses have limitations, there is a need for additional studies in other settings as archival World War One records become digitalised.