The use of immuno-double-diffusion tests in epidemiological studies of influenza in Spain

The immuno-double-diffusion (IDD) test was used to detect antibodies against the prevalent influenza A (H3N2) viruses in sera collected weekly in Madrid from 1972 to 1975. A total of over 14 000 sera were tested. The proportion of sera positive in the IDD test was found to reflect the level of recent influenza A infection of the population.