Studies on Antigenic Variations of the Haemagglutinin and Neuraminidase of Swine Influenza Virus Isolates

The neuraminidases and haemagglutinins of 20 isolates of swine influenza virus collected between 1930 and 1967 were compared in neuraminidase-inhibition and haemagglutination-inhibition serological tests. All the isolates were serologically related to a greater or lesser extent in tests with either antigen. The antigenicity of the two antigens varied independently in different isolates. There was a slight, but statistically significant, drift in the antigenicity of both antigens during the period in which the isolates were collected.