Epidemiology of influenza during 1957--75 in the Union of Soviet Socialist Republics and the German Democratic Republic.

Analysis of clinical data collected by the epidemiological services in both countries showed that each shift or drift in the antigenic structure of the influenza virus resulted in epidemics in both countries. Depending on the degree of antigenic change, both shifts and drifts influenced the speed of epidemic spread and the time of occurrence and the intensity of the epidemics. However, the analyses did not reveal a direct relation between degree of antigenic variation and the attack rate or the severity of the epidemic. The mean attack rate in both countries was about 13% depending on the pathogenicity of the circulating virus. The severity of the epidemics varied in the two countries but the fourth epidemic after each shift was severe in both countries. The reasons for this and other features of the epidemiology are discussed.