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Relationship Between Virus
Neutralising Titre and Potency
VNT and ELISA
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A high correlation exists in cattle between antibody titres to FMD virus following vaccination and resistance to challenge. A very substantial body of data has been generated by many independent authors.
To determine vaccine potency by serology, a group of animals is vaccinated with a field dose of vaccine and serum samples are usually taken at 21 days. Sera are then examined by the virus neutralisation test (VNT) or by liquid phase blocking ELISA.
The VNT measures the ability of a serum to prevent 100 tissue culture infectious doses of FMD from infecting BHK-21, IB-RS2 (porcine cell line), or other cells shown to be highly susceptible to the virus. In practice, the serum titre is calculated from the serum dilution which neutralises a fixed percentage of the virus infectivity (e.g., 50%, 90%) and the level of protection interpolated from previously established correlations. The 'Virus Neutralisation Test' and 'Serum Neutralisation Test' are different names for the same test. Strictly speaking, the former is grammatically correct but the use of the latter is commonplace.
Alternatively the sera may be examined by ELISA. This test is cheaper and quicker but measures all antibody to FMD virus rather than just neutralising antibody. The test has been used extensively and successfully in South America for potency testing of FMD vaccines.
The key features of serology are that it measures a relevant antibody
population and permits a comprehensive examination of the immune response
of a target species to different strains within a vaccine and over a period
of time rather than 21 days post vaccination alone. It also permits analysis
of the suitability of a vaccine to protect against field isolates.
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