Spatio-temporal investigation of the 1918 influenza pandemic in military populations indicates two different viruses

Abstract

There were multiple waves of influenza-like illness in 1918, the last of which resulted in a highly lethal pandemic killing 50 million people. It is difficult to study the initial waves of influenza-like illness in early 1918 because few deaths resulted and few morbidity records exist. Using extant military mortality records, we constructed mortality maps based on location of burial in France and Belgium in the British Army, and on home town in Vermont and New York in the USA Army. Differences between early and more lethal later waves in late 1918 were consistent with historical descriptions in France. The maps of Vermont and New York support the hypothesis that previous exposure may have conferred a degree of protection against subsequent infections; soldiers from rural areas, which were likely to have experienced less mixing than soldiers from urban areas, were at higher risk of mortality. Differences between combat and disease mortality in 1918 were consistent with limited influenza virus circulation during the early 1918 wave. We suggest that it is likely that more than one influenza virus was circulating in 1918, which might help explain the higher mortality rates in those unlikely to have been infected in early 1918.

Enhanced risk of illness during the 1918 influenza pandemic after previous influenza-like illnesses in three military populations

“…Of note in this regard, AIF soldiers who were medically treated for ARIs during April-July 1918 ('pandemic first wave') had similar illness rates, but were much less likely to die than their counterparts (OR 0•37, 95% CI 0•25-0•53, P < 0•001) during late 1918-1919 [5]. The simplest explanation of the findings is that at least two influenza virus strains circulated during 1916-19; this conjecture is consistent with recently reported genomic data showing multiple lines of influenza were present in 1918 [10] as well as a spatio-temporal study of both pandemic waves in the British Armies in France in 1918 [9]. These data coupled with the lack of protection from illness in late 1918 by ILI in early 1918 in both the AIF and USMA suggest that at least two immunologically distinct influenza viruses were circulating in 1918.…”

“…For example, compared to their urban counterparts, US soldiers who resided in rural areas of Indiana and Kansas prior to the pandemic had much higher influenza/pneumonia-related death rates in late 1918–1919. It is likely that residents of urban areas were exposed to greater numbers and more antigenically diverse respiratory infectious agents prior to entering military service in mid-1918 [8, 9].…”

Rural and Urban Differences in COVID‐19 Prevention Behaviors

“…Historically, rural populations have consistently shown higher mortality rates from infectious disease outbreaks and pandemics 17,18 . In a study examining military records during the 1918 pandemic (Spanish Flu), it was noted that rural soldiers were at a higher risk of mortality despite less social interaction than their urban counterparts 19 . More recently, excess mortality from infectious disease increased significantly for the rural poor through the 1990s and early 2000s 20 .…”

A Missed Summer Wave of the 1918–1919 Influenza Pandemic: Evidence From Household Surveys in the United States and Norway

“…Some isolated areas that were not affected in 1918 or 1919 experience their first encounter with the pandemic in a fourth bout during the winter of 1920, often with devastating consequences [ 7 ]. Although it is established that the H1N1 pandemic virus was responsible for both the summer and fall waves in the United States in 1918 [ 8 ], it is still unclear whether the herald wave during the spring of 1918 was caused by the pandemic virus or whether it was caused by a seasonal influenza or another endemic virus [ 6 , 9 ].…”