Example 8.1.3 of the textbook and the associated BB material on this website explains how an assay called the hemaglutination inhibition (HI) assay can be used to measure the binding strength between an influenza virus and an antiserum. The BB material from Example 8.1.3 also explains how such data can then be transformed into a measure of the antigenic difference between each virus and each antiserum. You can download this transformed data set from the link on the left. Each row in the data set represent a different virus and each column represents a different antiserum. The numerical values in the table are measures of antigenic difference between each virus and each antiserum as explained in the BB material from Example 8.1.3.
The plot below displays these antigenic difference data in three-dimensional antigenic space for different choices of antisera. Each point in the plot represents a virus, and the coordinates of the point represent the antigenic difference between that virus and the antisera on the coordinate axes.
Plotting the antigenic difference between viruses and antisera provides a helpful way to visualize changes in the antigenicity of viruses from one year to the next. For example, we can view the distance between viruses from 1969 (the blue points above) and viruses from 1976 (the grey points above) as the amount of antigenic change that has occurred between 1969 and 1976. A large distance means that a large amount of change has occurred.
The above kind of measure of antigenic change is important for designing influenza vaccines. The flu vaccine used in any given year will be protective against only a specific type of flu virus. As can be seen in the above data, the antigenic “profile” of circulating flu viruses changes from one year to the next. Scientists update the flu vaccine once the amount of antigenic change exceeds a certain threshold (Fouchier and Smith 2010).
References
Smith, D.J. et al. 2004. Mapping the antigenic and genetic evolution of influenza virus. Science 305: .371-376
Fouchier, R.A.M. and D.J. Smith. 2010. Use of antigenic cartography in vaccine seed strain selection. Avian Diseases 54:220-223