The spread of H5N1 influenza and the similarity between this avian virus and the Spanish flu virus causes fear of a new influenza pandemic, but data from the Spanish flu may also be of guidance in planning for preventive measures. Using data on influenza cases, influenza deaths and total deaths for Denmark and for Danish towns from 1917 to 1921, and population data from the 1916 and 1921 censuses, we analysed incident cases, cumulative, age-specific and age-standardized rates. Overall, more than 900,000 persons contracted flu during the y 1918-1920, and 1 out of 50 patients died from the disease. An early wave of the flu occurred in the capital and major towns, but not in peripheral towns. Influenza incidence in 1918 peaked at age 5-15 y, closely followed by the age groups 1-5 y and 15-65 y, but the influenza mortality was highest in the age groups 0-1 y and 15-65 y, with a peak mortality at age 20-34 y producing a W curve for mortality by age. The background for the better outcome in children aged 1-15 y as well as for the disease immunity in the elderly population should be further investigated.
Keywords: Adolescent; Adult; Age Factors; Aged; Aged, 80 and over; Child; Child, Preschool; Denmark; Disease Outbreaks; Geography; History, 20th Century; Humans; Incidence; Infant; Infant, Newborn; Influenza A Virus, H1N1 Subtype; Influenza, Human; Middle Aged