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Production Flow Chart
Development of Vaccine Strains
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Vaccine strains are derived from field isolates which are supplied free of extraneous agents by the World Reference Laboratory for FMD. The field isolates are given one or several passages in a cell line such as BHK-21 (baby hamster kidney) cells. The number of passages is limited as much as possible to minimise antigenic drift of the vaccine strain away from the field isolate. Important selection criteria include the ability of the new strain:
a) to adapt quickly and grow well in the production cells.
b) to induce high levels of antibody which strongly neutralise the field isolates, often with an r1 value of 1.
In practice, a vaccine strain rarely shows uniform homology with all field isolates from a particular episode, even though the field isolates may have been collected over a short period of time. The reason for this is that an FMD outbreak generates a population of viruses and individual isolates may vary slightly in their antigenicity from the isolate used to derive the vaccine strain.
Some vaccine strains prove to be highly immunogenic for reasons which are not known and are favoured because they are both excellent as vaccines against homologous challenge and more capable than less immunogenic strains of protecting against heterologous challenge, provided the relationship between the vaccine strain and the field isolate is not too distant.
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