Incidence of influenza C virus in Czechoslovakia and German Democratic Republic.

Out of 1091 and 2275 influenza virus strains isolated in Czechoslovakia (C.S.S.R.) and German Democratic Republic (G.D.R.) from 1969 to 1981, six and two were type C strains. All type C strains were isolated in the course of type A or B influenza epidemics. Double-diffusion tests indicated a relationship between the type C strains isolated in both countries in 1981 and the prototype strain C/Taylor/49; however, haemagglutination inhibition test (HIT) showed a heterogeneity within the 1981 group of viruses and their antigenic distinctness from the strain C/Taylor/49. The low number of type C isolates was in a sharp contrast with the antibody titres found in most adult sera. Only 10-30% of 275 and 2750 serum samples collected from persons aged 6-50 years in Czechoslovakia and G.D.R., respectively, were negative. The percentage of positive sera rose steadily in children 2-4 years of age. These serological findings suggest that influenza C virus was more or less permanently circulating among population, but the disease caused by it tends to take an inapparent or very mild course, so that the virus was isolated only exceptionally.