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

Background.  Reanalysis of influenza survey data from 1918 to 1919 was done to obtain new insights into the geographic and host factors responsible for the various waves. Methods.  We analyzed the age- and sex-specific influenza morbidity, fatality, and mortality for the city of Baltimore and smaller towns and rural areas of Maryland and the city of Bergen (Norway), using survey data. The Maryland surveys captured the 1918 fall wave, whereas the Bergen survey captured 3 waves during 1918-1919. Results.  Morbidity in rural areas of Maryland was higher than in the city of Baltimore during the fall of 1918, that was almost equal to that in Bergen during the summer of 1918. In Bergen, the morbidity in the fall was only half of that in the summer, with more females than males just above the age of 20 falling ill, as seen in both regions of Maryland. In contrast, more males than females fell ill during the summer wave in Bergen. Individuals <40 years had the highest morbidity, whereas school-aged children had the lowest fatality and mortality. Conclusion.  A previously unrecognized pandemic summer wave may have hit the 2 regions of Maryland in 1918.