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Opinion
Influenza
INTRODUCTION
One of the most difficult unanswered questions regarding the influenza pandemic of 1918-1919 is why very heterogeneous mortality rates were seen in very similar populations [1, 2]. Nearly identical military units operating in the same area had mortality rates varying by a factor of [= or >, slanted]10 and mortality rates on the Pacific Islands differed by a factor of nearly 50 despite all apparently being susceptible to infection by the pandemic influenza virus [3, 4]. Waves of influenza-like illness (ILI) that occurred in early 1918 resulted in much fewer deaths than the major wave that occurred in late 1918. The later wave is known from historical pathology specimens tested for surviving nucleic acids to have been caused by a H1N1 virus and resulted in the deaths of about 50 million people [5]. A possible explanation for the differences in numbers of fatalities is that the wave occurring in early 1918 was caused by a different influenza virus than the lethal wave of late 1918. The question then is whether there are differences between the early and late waves of 1918 that can be documented as possible evidence that two separate viruses were involved.
Nearly a century after the events one has to work without direct physical evidence of the virus of the early wave of 1918. The early wave sickened many but killed very few persons even in crowded military recruit camps such that very little archived pathological material exists. What autopsy material that has been studied indicates is, at least in some cases, a virus very much like that recovered late in 1918 was present as early as May 1918 [6]. In US Army recruit camps the early 1918 wave peaked in March and was gone by May such that the identity of the virus of the early 1918 wave remains unknown [7].
Because the 1918 pandemic was unanticipated, reliable prospectively collected ILI morbidity information prior to the main pandemic mortality wave is rare, especially on an individual rather than group basis. Those data that do exist are inconsistent; some indicating that ILI earlier in 1918 gave substantial protection against illness during the main mortality wave in late 1918, and some indicating there was...