Comparison of enzyme-linked immunosorbent assay and hemagglutination inhibition in a seroepidemiological study of influenza type C infection.

An enzyme-linked immunosorbent assay was developed for the detection of antibodies to influenza type C. Based on replicate testing it was decided that an increase of more than three times the standard error of the mean from one serum specimen to the next represented a significant increase in antibody content. After removal of potentially chicken egg albumin antibodies, the test appeared to be more sensitive for the detection of rises in antibody levels than was hemagglutination inhibition. In some cases, heat treatment of the serum before use in the enzyme-linked immunosorbent assay increased the readings. The strains C/AA/1/59 and C/NJ/1/76 were used in a pilot epidemiological study of influenza C infection in Tecumseh, Mich. Sera tested were collected from autumn of 1976 to autumn of 1978. The overall incidence rate for influenza C infection in the 2-year period was 7.8%. Most cases occurred in children aged 5 to 9 years, and the rates decreased rapidly with increasing age. There appeared to be no relationship to influenza C infection with influenza A or B coinfection. All 17 influenza C infections observed occurred during the 1976 to 1977 respiratory disease season. This indicates that type C virus may not occur endemically, but may exhibit year-to-year variation in infection frequency, as is the case with types A and B.