title: Enhanced risk of illness during the 1918 influenza pandemic after previous influenza-like illnesses in three military populations.
abstract: The reasons for the unprecedented mortality during the 1918 influenza pandemic remain poorly understood. We examined morbidity records from three military cohorts from years prior to and during the 1918 pandemic period to assess the effects of previous respiratory illnesses on experiences during the pandemic. Clinical registers and morbidity lists were examined to identify all medical encounters for acute respiratory illnesses in students at two U.S. military officer training academies and Australian soldiers deployed in Europe. Influenza-like illness prior to the major pandemic wave of 1918 predisposed Australian soldiers [relative risk (RR) 1·37, 95% confidence interval (CI) 1·18-1·60, P < 0·0001] and US officer trainees at West Point (RR 3·10, 95% CI 2·13-4·52, P < 0·0001) and Annapolis (RR 2·03, 95% CI 1·65-2·50, P < 0·0001) to increased risks of medically treated illnesses in late 1918. The findings suggest that susceptibility to and/or clinical expressions of the 1918 pandemic influenza virus depended on previous experiences with respiratory infectious agents. The findings are consistent with observations during the 2009 pandemic in Canada and may reflect antibody-dependent enhancement of influenza infection.
authors.affiliation: Australian Army Malaria Institute,Enoggera,Australia.
authors.forename: G D
authors.initials: GD
authors.lastname: Shanks
authors.affiliation: Department of Chemistry,U.S. Military Academy,West Point NY,USA.
authors.forename: S A
authors.initials: SA
authors.lastname: Burroughs
authors.affiliation: Department of Chemistry,U.S. Naval Academy,Annapolis,MD,USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Sohn
authors.affiliation: Department of Chemistry,U.S. Military Academy,West Point NY,USA.
authors.forename: N C
authors.initials: NC
authors.lastname: Waters
authors.affiliation: Department of Chemistry,U.S. Naval Academy,Annapolis,MD,USA.
authors.forename: V F
authors.initials: VF
authors.lastname: Smith
authors.affiliation: University of Queensland,School of Public Health,Brisbane,Australia.
authors.forename: M
authors.initials: M
authors.lastname: Waller
authors.affiliation: Armed Forces Health Surveillance Center,Silver Spring,MD,USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Brundage
mesh_terms: D000293:Adolescent; D001315:Australia / Q000453:epidemiology; D005060:Europe / Q000453:epidemiology; D049673:History, 20th Century; D006801:Humans; D007251:Influenza, Human / Q000453:epidemiology / Q000266:history* / Q000821:virology; D008297:Male; D008396:Maryland / Q000453:epidemiology; D008889:Military Personnel / Q000706:statistics & numerical data; D009518:New York / Q000453:epidemiology; D058873:Pandemics / Q000266:history*; D012306:Risk; D055815:Young Adult
keywords: 1918 pandemic; influenza; military; mortality